The effect of cellobiohydrolase I on low consistency refining and handsheet properties

Abstract

Papermaking is an energy-intensive process, with mechanical pulp refining accounting for 30% of the total electrical energy consumption during papermaking. Presently, with significant economic and environmental concerns, improving energy efficiency and minimising the environmental impact of paper manufacturing is critical. Biotechnological treatments of pulp with enzymes have demonstrated a high potential for lowering energy consumption and greenhouse gas emissions. The benefits of enzymes include energy savings, environmentally friendly treatment, and improvement in fibre and pulp quality, while requiring only modest adjustments to the existing industrial processes. At present, the most investigated enzymes for fibre modification are cellulases. This research aimed to investigate the effect of the cellulase, cellobiohydrolase I (CBH I) as a refining enzyme. Three structurally different pulps were investigated: a bleached hardwood kraft pulp (BHKP), a fully bleached softwood (FBSW) pulp, and an unbleached softwood (UBSW) pulp. The research project was divided into three phases. In the first phase, CBH I was characterised by determining the effect of temperature and pH on CBH I activity using a filter paper assay. The thermostability of CBH I was then analysed, using the optimal temperature and pH. In the second phase, the use of CBH I as a pre-treatment on the pulp before refining was investigated using the bijective diagram technique. A 12” single-disc pilot refiner was used to simulate industrial conditions. The effect of varying the Specific Edge Load (SEL) and refiner plate design for the CBH I treated pulp was evaluated against the reference pulp with CBH I (REF CBH I). The fibre morphology, handsheet properties, and energy consumption were evaluated at a specific desired Canadian Standard Freeness (CSF) for each pulp. In the third phase, the refining data obtained in the second phase was modelled using customised refining software to determine the optimum refining parameters required for the best handsheet strength properties with minimum energy consumption. The software-generated optimum refining parameters were validated by refining each pulp three times under the specific conditions. Statistical analysis was used to test the repeatability of the three runs for each pulp. The first phase showed that CBH I is a thermophile enzyme with an optimum temperature of 55 °C. The enzyme works best in acidic conditions, with an optimum pH of 4. Furthermore, CBH I is thermostable, with 49% of its activity retained at the end of a 3 h incubation. In the second phase, CBH I pre-treatment resulted in energy savings of 33%, 10%, and 22% for BHKP, FBSW, and UBSW, respectively. All three treated pulps responded better to low intensity (SEL-). In contrast, increasing the refining intensity (SEL+) deteriorated the pulp

quality for all three pulps. Changing the refiner plate design led to a different result for each pulp. For the BHKP, the GC run (plate design with smaller bar width and grooves) led to the highest strength development, but also required the most refining energy. Whereas the GA run (plate design with a bigger bar angle) led to more fibre cutting than fibrillation. For the FBSW, both the GC and GA plate designs improved the fibre development. On the contrary, the GA and GC plates for the UBSW resulted in poor fibre development. In the third phase, the optimum plate designs generated by the model for the BHKP, FBSW, and UBSW were REF, GA, and REF plate designs, respectively. For all three pulps, one-way ANOVA analysis showed a statistically insignificant difference between the pulp properties of the three runs conducted for each pulp. In conclusion, the CBH I enzyme has the potential to transform the refining process towards a more environmentally friendly, cost-effective, and efficient operation compared to the conventional method.