Digester Scale Control:  Fundamentals and Best Practices for Responding to Process Variations, 2018 PEERS

Kraft cooking conditions provide an ideal environment for the formation of calcium carbonate scale.  Calcium carbonate deposits can negatively affect the pulping process, pulp production, pulp quality, heater and digester cleaning time and frequency, and worker safety, thereby significantly increasing mill operation costs.  The financial impact of an ineffective digester scale control program for an average-sized pulp mill may exceed $1 million a year. 

During major shutdowns, mill management typically re-evaluates its existing approach to managing scale in the digester.  This can lead to initiation of a new chemical treatment program, adjustment to an existing program or a change in program supplier.  Typically, the chemical treatment program cost is the dominant decision criterion.  However, there are additional operating costs associated with an ineffective digester scale control program. Selection of digester scale control program should be compared with the total costs of a scale control program.  A full understanding of the costs and benefits of different scale control programs will help mill determine the most cost-effective options. 

Process variation is a common theme, and many process parameters affect scale formation. Some process parameters have a much greater influence on scaling potential than others.  Understanding these influential parameters will enable a mill to minimize the impact of these parameters on a scale control program.  This paper discusses how each process parameter affects the scale control program and how to design a digester scale control program that minimizes the impact of process variations to meet the challenges of a specific digester and/or the entire pulp mill.  Additionally, case studies provide examples of cost-effective scale control programs. 

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