Controlling of chemical recovery and causticizing process using on-line analysers

摘要

Production of cellulose in the sulphate cooking plant is accompanied by the recovery of Green Liquor. On-line measurements control this causticizing step with financial and production benefits. Metrohm Applikon has built and installed several on-line monitoring analysers to control causticizing process. In the presentation, all studies were made in Scandinavia, Northern Europe. Measurements in the chemical recovery process include: Effective Alkali (EA), Active Alkali (AA), Total Titratable Alkali (TTA) and the degree of causticizing. These measurements were made according Scandinavian pulp, paper and board standards. EA, AA, TTA and the degree of causticizing were measured according standard SCAN N30:85 in green liguor, lime milk and white liquor. Measuring sulphate concentrations was made according standard SCAN N6:85 in green liquor. Sulphate content was also measured with the latest innovation method to measure sulphate content in green liquor using rapid thermometric titration. Thermometric titration measures enthalpy, temperature changes, in the reaction and can be used with several endothermic and exothermic reactions. Study includes description of on-line analysis methods according SCAN N30:85 and SCAN N6:85 standards. In the study useful sampling points are presented in chemical recovery to measure EA, AA, TTA, degree of causticization and sulphate. The sampling was fully automated and all the samples were settled before taken into the analyser in the special settler. In the analyser samples were titrated with potentiometric and thermometric titration. Titrations were performed in the ventilated wetpart cabinet. During titrations hydrogensulphide gas is produced and for a safety reason it was removed automatically. After analysis's automated cleaning cycles were executed regularly to prevent blockages in the system. Calculated results were sent to the customer distributed control system (DCS) with the Profibus protocol and current message (mA). Design requirements as well as material requirements to build workable pre-sampling systems are presented based on several field installations. With the automated on-line analysers maintenance and service play a very important role. In the study normally needed maintenance and service with on-line analysers are presented. Typical results and titration curves based on several on-line studies and field installations are presented in the study including repeatability and inaccuracy of the analysis results using on-line analysers.