Comprehensive Wet End Control

Papermakers have three prevailing objectives: quality, runnability and overall performance. Today, the overall performance means aiming to achieve the best possible energy efficiency and energy saving in the whole production line. Securing runnability and quality in all situations is an effective way to achieve this. To meet this, it is vital to stabilise consistency, ash content, charge and chemistry at start-ups, during normal running and after grade changes. It is best done by measuring, analysing, controlling and managing the wet end as comprehensively as possible. This paper presents two superior tools for wet end management: a modular multi-talent analyser and consistency sensor, and a multivariable control system utilising the measurements. The analyser is the first in the world that provides papermakers with all relevant wet end measurements. It performs a multitude of measurements and analyses: charge (ionic demand), chemistry (pH, temperature, conductivity, and redox), total and filler consistency, total and filler retention, filler content, and turbidity. The consistency sensor is a stand-alone device that performs continuous measurements from one sample point. It is ideal for applications that require continuous consistency measurement and automatic control. The benefits of wet end control in today's processes is indisputable. Wet end control solutions are common practice on the modern and most efficient paper machines operating today. Retention control stabilises white water consistency by controlling the retention chemical flow. Ash control effectively eliminates furnish filler content variations entering the the wet end. Charge monitoring and control are important in creating optimum conditions in the wet end chemistry. The correct charge level also ensures proper retention and web formation. When these controls together are optimised, productivity reaches a level that is not possible with individual control. The measurements complement Metso Automation's offerings for wet end and paper quality management. It makes it possible to integrate dry and wet end measurements and controls into one true multivariable model predictive (MPC) control system. The multivariable control takes into account interactions between white water consistency, paper ash and oven dry weight controls. The benefits of comprehensive wet end control are clear and can be seen in less breaks, more stable dewatering leading to good runnability and stable quality, a cleaner machine and chemical savings.