The New Approach To Paper Chest Mixing

In the pulp & paper industry, stock chests with mixing and recirculation loops are used to reduce the magnitude of consistency fluctuations in pulp production. Minimizing these fluctuations can provide a substantially smoother, more uniform surface and significantly improve final sheet properties resulting in more first-quality tons and improved savings/revenue. This result, which is generally acknowledged, is highly dependent on stock chest mixing/dilution.

Mixing in typical machine chests is marginal and limited only to the chest bottom. With limited mixing the chest output, consistency is typically controlled by recirculation loops. The automatic consistency control loops have relatively long-term constants and can not react to short-term consistency fluctuations. The solution to paper stock consistency fluctuations is to create zonal mixing that also improves residence time distribution (RTD) and prevents the short circuiting of flow within stock chest.

The common perception is that the stock chests mixing equipment carries a high price tag. This misconception has prevented full-scale experimentation in the past. Due to the complex rheology of paper pulp, numerical simulations and small-scale experiments have not been fully persuasive. The challenges associated with scale-up have proved especially controversial for engineers and managers.

Proper chest mixing can be accomplished by utilizing a correctly sized top entry agitator with new impeller technology that offers 50% power savings over the more commonly used side entry mixers.

This paper discusses the rheology and scale-up issues that have created the demand for successful, continuous full-scale reactors with numerous mixing zones. The cost of the installation of a full scale mixer can potentially be returned in as little as one month of operation.