STAGEWISE COUNTERCURRENT WASHING OF CHEMICAL PULP: A MULTICOMPONENT MASS TRANSFER ANALYSIS

Abstract

The washing of chemical pulp in a countercurrent cascade of rotary vacuum filters is described in terms of a model of the multicomponent mass transfer process occurring in the wash zone of each stage. The classical Norden Efficiency Factor method is extended to the multicomponent case. Relationships among the system loss ratios, wash and recycle ratios, and the number of stages are developed. In the paper industry, chemical pulps are washed to recover cooking chemicals and to obtain satisfactory quality for subsequent processing. With the present increased emphasis on pulp mill closure, the industry is attempting to reduce losses from brown stock washing systems. This leads to the need for improved engineering models of the washing operation for either design or control purposes. Countercurrent washing, either stagewise or continuous, is generally used to obtain good recoveries at relatively low dilution ratios. In the stage-wise mode, rotary vacuum filters are commonly employed with reslurrying of the stock between stages. The important chemical species in this separation process are sodium and lignin. In addition, other attributes, such as color and BOD₅, are of increasing importance with respect to the performance of pulp washing systems. Aggregate characteristics, such as total dissolved solids (TDS) are also of concern. This paper considers stagewise countercurrent washing of pulp in a cascade of rotary vacuum filters. A model of the multicomponent mass transfer process occurring in the wash zone of each stage is developed and incorporated into the system material balances to produce results useful for either design or performance calculations.