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

摘要

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 ton 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 chemical, steam, and power. The mill's economic viability rests in large part from best utilization and efficient and optimized chemical recovery and pulping operations. The use of Near Infrared Spectroscopy (MRS) has been fully explored and developed by various research and development groups around the world and some have now been successfully implemented as online analyzers for process control in pulping operation, including: raw green liquor for TTA control, clarified green liquor for TTA trim control and feedforward slaker control, causticizer white liquors for CE control, as well as weak black liquor residual effective alkali, lignin, organics, inorganics, and total solids content to provide feedforward for evaporator control. Measurements of woodchip properties as well as extracted wood core samples from standing trees are also realized. These measurements have led to new advances in pulping operations as well as new control strategies which 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 realized. With implementation of these analyzers and control strategies over the past three years, mills have reported significant process variability reduction, typically from 40% to 60%, allowing for target shifts to more optimized operation. Chemical savings and energy cost reductions have all been reported, along with lower maintenance costs, reduced plugging in evaporators and recovery boiler, leading to increase runnability. In some cases, debottlenecking recausticizing and recovery boiler unit operations resulted in significant increase in digester production. ROI ranged from $500K to $3M. This paper briefly reviews the technology and economic impacts achieved through fibre value chain maximization from adoption and implementations of process analyzers and automated process control to achieve efficient and optimized pulping and chemical recovery operations.