800xA APC for continuous pulp digester
Advanced process control (APC) using straightforward design and deployment of model predictive control (MPC) with System 800xA DCS enables higher level of automation and optimization of continuous cellulose fiber or pulp output
The Kamyr continuous pulp digester is a complex tubular reactor where wood chips react with an aqueous solution of sodium hydroxide and sodium sulfide (referred as white liquor) to remove the lignin from the cellulose fibers. The product output from the digester is cellulose fibers or pulp.
Most continuous digesters consists of three basic zones: an impregnation zone, one or more cooking zones and a wash zone. The white liquor penetrates and diffuses the wood chips as it flows down through impregnation zone. The mix is heated to a target cooking temperature where bulk delignification starts, and the majority of lignin is removed. The cooking process is stopped at the beginning of the wash zone by reducing the temperatures and cooked pulp is washed in a counter-current washing zone, using wash liquor injected at the bottom of the digester.
Producing an even quality pulp at a consistently high production rate is a challenging task for digester operators where the raw material quality, such as chip size and chip moisture, tends to change with the seasons, natural geographic factors, and the wood source. The schedule also swings from hardwood to softwood making the process control task more complex.
At the core, advanced control package for the digester (known as OPT800 Cook/C, an application built on the System 800xA APC platform) stabilizes pulp production, reduces chemical usage and coordinates the numerous loops to incur optimum, on-specification, pulp quality at minimum variance. This optimum pulp quality production assures the minimization of bleaching chemical use where bleached grades are produced.
In addition, these controls maximize production, yield, and paper stock drainage on the paper machine in both bleached and brown product mills. The process variables includes blow kappa number, digester level, residual alkali concentration and the production rate. The product quality is specified in terms of Kappa target, and level range.