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Ind. Eng. Chem. Res. 2009, 48, 9830–9836
Bleaching Miscanthus x giganteus Acetosolv Pulps with a New Totally Chlorine-free Sequence and the Effect of Carbohydrate Protectors Juan Jose´ Villaverde, Pablo Ligero, and Alberto de Vega* Department of Physical Chemistry and Chemical Engineering, UniVersity of A Corun˜a, 15071 A Corun˜a, Spain
The bleaching of semibleached Miscanthus x giganteus Acetosolv pulp, after alkaline and peroxyacetate treatments, was investigated by a novel totally chlorine-free (TCF) sequence. Oxygen delignification (Ostage), peroxide (P-stage), and oxygen-pressurized hydrogen peroxide stage (PO-stage) were assayed in order to reach the target values for dissolving pulps. Using factorial experimental designs, we investigated the effects of the bleaching variables on the parameters defining the process and checked the effects after adding chelating agents. Regression equations satisfactorily fitted the experimental results, and under the optimum conditions, a fully bleached pulp (86.9% ISO brightness) with a viscosity of 786 cm3/g and a very favorable kappa number (1.3) was obtained. Carbohydrate protectors substantially improved the characteristics of dissolving pulps, but pulps with kappa values above 6 did not improve with carbohydrate protectors as expected. Introduction One of the most pressing challenges of the 21st century will be the development of a sustainable supply of sources of energy that do not cause long-term environmental change. Biomass (nonfood cellulosic plant material) is the primary candidate for a variety of environmental and economic reasons, including its chemical diversity and the large and renewable supply at low cost. A model of sustainable development based on biomass will require the development of three biobased sectors: biopower/ bioenergy, biofuels, and biobased products, pillars of the called bioeconomy.1,2 However, biomass processing technologies are still in a relatively nascent stage and the development of the biorefinery represents an important step. Cellulose produced by lignocellulose feedstock (LCF) biorefineries1 may be the most important raw material in the future biobased economy owing to the production of bulk chemicals, new materials, fuels, and energy. Miscanthus x giganteus Greef and Deuter ex Hodkinson and Renvoize,3 has attracted the interest of the EU authorities as a biomass source, and in 1993, a European Research Project (Miscanthus Productivity Network: AIR1-CT92-0294)4 was implemented to evaluate the potential of this grass as a biomass source in Europe as well as its use in a wide range of applications within the emerging concept of the biorefinery. Cellulose pulps obtained by acid organosolv fractionation processes from M. x giganteus5,6 open a very promising path for LCF organosolv biorefineries. Dissolving pulp is a well-suited raw material for different kinds of cellulose derivatives, such as carboxymethylcellulose, cellulose acetate, cellulose nitrate, and various types of regenerated cellulose such as viscose and microcrystalline cellulose.7 Total chlorine-free (TCF) bleaching processes are environmentally friendly technologies, capable of producing chemical pulps with low residual lignin content, although less efficiently than with elemental chlorine or elemental chlorine-free (ECF) bleaching technologies. The aim of this study is to verify the possibility of producing dissolving pulp (from Miscanthus Acetosolv pulps) using a new TCF optimized sequence, to investigate the influence of bleach* To whom correspondence should be addressed. Tel.: +34981167000. Fax: +34-981167065. E-mail:
[email protected].
ing process variables in O-, PO-, and P-stages and to check the influence of chelating agents in the bleaching sequence. Materials and Methods Raw Material. M. x giganteus from an experimental plantation established as part of a European Research Project (Miscanthus Productivity Network. AIR1-CT92-0294)4 near Santiago de Compostela (Spain) was used in this study. The stalks were manually stripped of leaves, opened longitudinally, and depithed. The depithed stems were milled, sieved, and the fraction with particle sizes smaller than 1 cm was used in the experimentation. After a last drying period, the stems were hermetically stored in polypropylene containers. Characterization of the Raw Material and Pulps. For all experiments, kappa number (KN), intrinsic viscosity (VIS) and ISO brightness (BR) were measured according to TAPPI standards (T 236, T 230, and T 525, respectively). Pulp yields (PY) were determined gravimetrically after oven drying until a constant weight was obtained. Saponifiable groups (as NaOH equivalent) were performed by KOH-ethanol saponification.8-10 Acetosolv Fractionation. A mixture of ground M. x giganteus bark, water, and acetic acid (weight percentage with respect to cooking liquor ) 90), were heated to boiling in a 3000 cm3 glass reactor. Hydrochloric acid (weight percentage with respect to cooking liquor ) 0.15) was added when the boiling started, and the mixture was refluxed with stirring at atmospheric pressure for 55 min and a liquid/wood ratio of 12. The pulp was then filtered, treated four times with 85% acetic acid (in w/v proportions of 0.4, 0.4, 0.2, and 0.2 with respect to the initial dry weight of M. x giganteus), and washed repeatedly with distilled water to neutrality. The process and reaction conditions used here have been optimized in previous works.6 TCF Bleaching. The bleaching tests with alkaline treatments (E-stages), peroxyacetate treatments (Pab-stages), and peroxide bleaching (P-stages) were carried out at the desired consistency, in sealed polyethylene bags, immersed in a thermostatic water bath at the desired temperature. Samples were kneaded several times during the reaction. The process and bleaching conditions, used in E- and Pab-stages, have been optimized in previous works, and were carried out with the following optimal conditions:10(i) E-stage: 10% consistency, 60 min, 70 °C, and
10.1021/ie9008277 CCC: $40.75 2009 American Chemical Society Published on Web 09/02/2009
Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009
6% of sodium hydroxide; (ii) Pab-stage: 10% consistency, 30 min, 55 °C, 7% peroxide charge, and pH ) 11. Bleaching with oxygen (O-stages) and oxygen-pressurized hydrogen peroxide (PO-stages) were performed in a 0.5 L stainless steel Parr high-pressure reactor (model 4560, Parr Instrument Co., Moline, IL) under oxygen pressure at the desired consistency. A helicoidal rod was used to provide adequate stirring of mixtures. The reactor pressure was measured with a pressure gauge, and temperatures were controlled using a Parr controller (model 4842). After the treatments, the pulps were washed with an aqueous solution at pH 11 to keep the solution basic and thus prevent lignin precipitation. Experimental Designs and Statistical Analysis. Factorial experimental designs were chosen to examine and quantify the effect of the bleaching variables on the parameters defining the process in O-, P-, and PO-stages. Factorial designs have been used extensively to examine the behavior of bleaching with organosolV pulps.11-14 According to standard procedures in experimental design construction and analysis, the independent variables were normalized in accordance with
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