| The main objective of this research was to examine the effects of high pressure on spruce organosolv pulping behavior in terms of fiber liberation point (FLP), degree and selectivity of delignification, screened yield and residual lignin content of the resulting pulps at the FLP. In this research, batch, isothermal delignification of spruce wood chips was carried out by means of high-pressure neutral alkali earth metal (NAEM)-catalyzed organosolv pulping.; It was found that, in addition to cooking temperature and time, pressure had also a statistically significant effect on pulping behavior of spruce wood and on pulp chemical properties. Pulping was pressure sensitive particularly when cooking at low temperatures (190°--200°C) for short cooking times until the fiber liberation point was reached. Pressure was more effective in lowering reject content and consequently increasing screened-yield, than in reducing Kappa number. Thus, the issue of screen rejects could be resolved by applying high pressure (optimally 2000 psi) to the softwood organosolv pulping system while producing pulps of various qualities/grades that can be utilized for different applications.; Mathematical models were then developed, tested, validated and finalized. The final models can predict many properties of the softwood NAEM pulp from the cooking variables with high accuracy (R2 ≥ 92%) and thus can be applied for process control purposes in the pulp mill. The response variables modeled include total yield, reject content, Kappa number, residual and dissolved lignin, lignin-free yield, and mannose and xylose contents of the NAEM pulp. The general form of the models for most responses is as follows: Y = b0 - b1T + b2 Tt - b3t - b4 t1/2 - b5P, R2 = 92--99%.; Optimum cooking conditions to obtain early fiber liberation, low reject content (<2%), and high yield screened pulp were found to be: pressure (P) of 2000 psi for 40 min and 80 min (t) at 200° and 190°C (T), respectively. The resultant pulps had screened yields of 57.5 and 56.2% and Kappa numbers of 63 and 38, respectively, both with a lignin-free yield of >53%. Increasing the cooking temperature above 200°C is not recommended since it extensively dissolved hemicelluloses and degraded cellulose, resulting in a lower pulp yield and viscosity. (Abstract shortened by UMI.)... |
