Pulp mill and chemical recovery control with advanced analysers - from trees to final product

With increasing competition worldwide, pulp and paper manufacturers require greater emphasis on cost reduction and increased product efficiency and quality. Variable costs for producing one tonne of pulp include: fibre cost, chemical costs, energy costs, material and maintenance costs, and labour costs. For most mill operations, the highest cost is often the wood/fibre but little has been done to measure their properties. Once the wood enters the pulping processes, pulping chemicals, energy, process uptime/reliability (maintenance) and labour efficiency dictates the final production costs. In this, the chemical recovery process continues to be a critical component of a pulp mill, providing the chemicals, steam and power. The mill's economic viability rests in large part from best utilisation and efficient and optimised chemical recovery and pulping operations. The use of Near Infrared Spectroscopy (NIRS) has been fully explored and developed by various research and development groups around the world and some have now been successfully implemented as online analysers for process control in pulping operation, including: raw green liquor for TTA control, clarified green liquor for TTA trim control and feed-forward slaker control, causticiser white liquors for CE control, as well as weak black liquor residual effective alkali, lignin, organics, inorganics, and total solids content to provide feed-forward for evaporator control. Measurements of woodchip properties as well as extracted wood core samples from standing trees are also realised. These measurements have led to new advances in pulping operations as well as new control strategies that take advantage of the multitude of available properties, such as the potential yield of the forest to chip moisture content and complete pulping liquor chemical compositions. Advanced control strategies for digester, dissolving tank, slaker control and final CE control can be realised. With implementation of these analysers and control strategies over the past three years, mills have reported significant process variability reduction, typically from 40 to 60 per cent, allowing for target shifts to more optimised operation. Chemical savings and energy cost reductions have all been reported, along with lower maintenance costs, reduced plugging in the evaporators and recovery boiler, leading to increased runability. In some cases, de-bottlenecking re-causticising and recovery boiler unit operations resulted in a significant increase in digester production. ROI ranged from $500,000 to $3 million. This paper briefly reviews the technology and economic impacts achieved through fibre value chain maximisation from the adoption and implementation of process analysers and automated process control to achieve efficient and optimised pulping and chemical recovery operations.