Pulping of holm oak wood. Influence of the operating conditions Francisco Lo ´pez a , Joaquı ´n Alaejos b , Alejandro Rodrı ´guez c, * , Luis Jime ´nez c a Department of Chemical Engineering, University of Huelva, Huelva, Spain b Department of Agroforest Sciences, University of Huelva, Huelva, Spain c Department of Chemical Engineering, University of Co ´ rdoba, Co ´ rdoba, Spain Received 24 November 2006; received in revised form 17 January 2007; accepted 21 January 2007 Available online 12 March 2007 Abstract This paper reports on the influence of independent variables in the pulping of holm oak wood (Quercus ilex L.) [viz. temperature (135– 195 °C), cooking time (30–90 min) and soda concentration (10–20%)] on the yield, holocellulose content, a-cellulose content, brightness and viscosity of the resulting pulp. By using a central composite factorial design, equations relating each dependent variable to the different independent variables were derived that reproduced the experimental results for the dependent variables with errors less than 5–15% in all cases. The highest pulp yield (56.9%) was obtained with the lowest values of the operating variables. However, obtaining the optimum holo- cellulose content, a-cellulose content and viscosity (viz. 94.5%, 78.5% and 1395 ml/g, respectively) entailed using values of the indepen- dent variables above their mean levels. Also, ensuring optimal brightness (viz. 24.3%) required using higher temperatures and soda concentrations. A compromise that saves equipment immobilized capital and about 25% of soda is using a soda concentration of 15% at 195 °C for 30 min. The yield thus obtained differs by less than 29.5% from the highest level; also, the resulting holocellulose content, a-cellulose content and brightness differ by less than 12% from their respective optimum values. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Forest residues; Quercus ilex L.; Holm oak; Soda pulping; Pulp 1. Introduction Paper usage has grown steadily worldwide over the last few decades. Specifically, the amount of paper used yearly in the world increased from 1.025 · 10 6 to 1.625 · 10 6 ton between 1988 and 1998, (Bayer et al., 1999). In Europe, an increase of 41.4% has taken place in the consumption of paper from 1991 to 2005, spending from 62.387 · 10 6 to 88.213 · 10 6 ton (Annual Statistics, 2005). In the period 1995–2002, the paper production in Spain has registered an accumulated growth of 46%, (Aspapel, 2006). With the inception of new computer-based and audiovi- sual technologies, a gradual decrease in paper consumption was envisaged; rather, the expansion of computers has boosted paper usage, (Anuario de Estadı ´stica Agrario, 2003). This has raised the worldwide consumption of pulp- wood to 170.358 ton in 2003. The environmental problems raised by this increase in wood usage have elicited major technological efforts involving the development of new, faster-growing wood species of higher quality, as well as less polluting cooking and pulp bleaching processes; additional efforts have focussed on recycling used materials and partly replacing traditional wood raw materials (hardwood and softwood species, mainly) with non-wood and residual materials including trimmings from various tree species, biomass from various grass crops, agricultural residues and forest residues. These last have the additional advantage that they 0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.01.008 * Corresponding author. Address: Departamento de Ingenierı ´a Quı ´mica, Campus de Rabanales, C-3, Universidad de Co ´ rdoba, Co ´ rdoba, Spain. Tel.: +34 957 21 85 86; fax: +34 957 21 86 25. E-mail address: q42ropaa@uco.es (A. Rodrı ´guez). Available online at www.sciencedirect.com Bioresource Technology 99 (2008) 819–823