FAIR-CT97-3855 Comparative production of dissolving pulps by acetosolv, formacell, milox and steaming-extraction pulping

Contacts
Summary Information



To find similar Items, click on a keyword below:
FAIR Area 1.3 - Forestry-Wood Chain : Paper/Pulp : Pulping : Wood (Lignocellulose)

	Contract No:	FAIR-CT97-3855
	Date Prepared:	February 2001
	Source:	Final Report Executive Summary

---

Final Report Executive Summary

Introduction

The aim of this project was to carry out a thorough investigation of three new acidic pulping processes known as Acetosolv, Formacell and Milox. These are based on the use of organic acids as the pulping medium as an alternative to the well-known Sulphite process. The three new processes have been investigated on a comparative basis from the initial input of wood to the production of polymeric products. In general the new processes excelled with a higher selectivity for hemicellulose removal rather than cellulose degradation, when aspen, beech or eucalyptus were used as a starting material. Accordingly pulps of higher viscosity and purity could be produced. This was especially true for Milox- and Formacell-pulps.

Results

All unbleached pulps derived from the new processes were lower in brightness than comparable Sulphite pulps. Nevertheless the bleaching operation of organosolv pulps successfully led to material of high brightness. The highest selectivity in bleaching was achieved for the sulphite pulps, followed by Formacell- and Milox pulps. Acetosolv pulps had the lowest selectivity in bleaching operation. Nevertheless bleached pulps from all processes met the demands for further refinement in the viscose- and the NMMO-process. The prerequisites for conversion into cellulose acetate, which is probably the most demanding conversion process in the cellulose industry, could not be entirely fulfilled.

Structural investigations of pulps using ¹³C-CP/MAS-NMR spectroscopy, x-ray analysis (SAX, WAXS) and transmission electron microscopy revealed that the conformational order of cellulose from the sulphite and Milox process were higher than those of Formacell and Acetosolv pulps. The investigations on the pore structure of the new pulps gave a rather non-uniform picture. This finding was taken as an indication of the impact of the different bleaching processes resulting from the different pulping concepts. This was avoided by using a very similar bleaching sequence (EOZQP) for those pulps which originated from eucalyptus wood. Pulps from this raw material, which were obtained from the Milox and the Acetosolv processes gave the best results in acetylation experiments. However the performance of the pulps in the conversion into cellulose acetate cannot be described as fully satisfactory. Results of less satisfaction were also obtained for the Acetosolv- and Milox pulps, when their performances were tested in the viscose process. Formacell pulps however responded satisfactory in viscose processing and led to viscose fibres which were superior to those from sulphite pulping. Interestingly the strength properties of viscose fibres from organic acid pulping were clearly higher than those from sulphite pulps. This finding can be linked to the high proportion of cellulose IV (20 to 50%), which is a high temperature crystalline modification of cellulose. Normally viscose fibres are made up of cellulose II. Summarising these results it can be concluded that the Formacell process has a high potential for application in connection with the viscose process.

Bleached pulps from the three processes were also tested for their suitability in the NMMO fibre and film processing route. Generally it turned out that the processibility was guaranteed in all cases. Differences in structure and morphology of the pulps were of far less impact on the NMMO processibility than it was the case in the derivatisation processes. The attainable properties were widely similar to those obtained with commercial pulps.

By-products from sulphite pulping amounted to approximately 20% for beech, 17% for aspen and 15% for eucalyptus. The by-products included xylose, furfural, acetic acid, mannose and xylonic acid. In Formacell pulping organic acids in quantities of 4-6% were incorporated into the pulp, the lignin and the hemicellulose fraction. Accordingly a concept for the recovery of these acids under minimised consumption of alkaline is essential for the commercialisation of this process. The major by-product was lignin, which is obtained in yields between 19 to 25% (higher than the original lignin content due to condensation reactions with carbohydrates). While the utilisation of carbohydrates and low molar mass phenols seemed to be unattractive, furfural could be recovered in yields up to 3.5%, based on wood. This compound will be beneficial for economical evaluation of the process and should be included in optimisation of pulping conditions. Similar results were obtained for the Acetosolv-process, in which 20-40% of the initial xylan was converted into furfural. Xylose accounted for similar percentages of the initial xylan. Some xylan, originally present in the wood, could not be accounted for in the balance sheet and was probably converted into insoluble furfural-related condensation products. In Milox pulping up to 95% of the initial xylan was converted into xylose, furfural being a minor reaction by-product. In addition to xylose, significant amounts of acetic are generated as a by-products in Milox processing.

The major equipment of a Kraft or sulphite pulp mill, however, cannot be used for the new pulping processes because the construction materials are not sufficiently resistant to the organic acids, which can be considered as the major disadvantage. This is the reason, why a corrosion study was performed within the project in order to evaluate this problem.

During this evaluation it became clear that the impact of corrosion strain was by far the greatest in the Acetosolv process. The least impact on the tested materials was found in the Milox process. The comparison of mass losses and rates of corrosion proved that the material Hastelloy C had a clearly better corrosion behaviour. The material can be regarded as acceptable for use in the Formacell and in the Milox process on the basis of the assessment standard. The rates of corrosion observed are clearly higher when used in the Acetosolv process, but considerably lower than in the case of previously tested steel samples.

Conclusions

It has been shown that the Formacell process has a good potential to be developed as a new pulping technology for the production of dissolving pulps, especially where delignification selectivity could be improved by preventing competitive condensation reactions. Currently, the main disadvantages of this technology are:

• the high costs (28 Euro/t odbp for Eucalyptus Formacell vs. 15 Euro/t odbp for eucalypt Mg-Sulphite pulps) for the applied bleaching sequence (EOZQP)
• the need for expensive corrosion-resistant materials, and
• the high sensitivity towards wood species with a high content of parenchyma cells.

The simplicity of the recovery systems required, the absence of sulphur and other hetero-elements in the pulping liquor, the reasonable high pulp yield and the high potential for by-product recovery in combination with the excellent strength properties of the produced rayon fibres makes the Formacell process an interesting option as a future technology for the production of viscose pulp.