Wood contains mainly cellulose, hemicellulose, lignin and small quantities of extractives. The primary purpose of chemical pulping is to remove the lignin which binds the fibers in the wood and liberate the cellulosic fibers. Kraft pulping is the major chemical pulping process used by the pulp and paper industry for the liberation of cellulosic fibers from the structural matrix of wood. The kraft process uses sodium hydroxide and sodium sulfide as effective chemicals for pulping. The chemicals along with wood chips are charged into a pressure vessel (digester), and the contents are brought to the reaction temperature at a preset rate. After the digestion the chips are disintegrated by being "blown" into a tank and then sent for further processing. The pulp resulting from digestion and disintegration of the wood chips is washed thoroughly in a counter current system of washers to produce spent liquor or "black liquor". This black liquor is sent through a recovery cycle which involves concentration of black liquor to 60-70% solids by evaporation and burning of the concentrated black liquor in a furnace. In the furnace the organic materials are completely burned to generate steam, the sodium-sulfur compounds are reduced to sodium sulfide, and the remainder of the inorganic components are converted to sodium carbonate. The inorganic compounds are withdrawn as smelt from the furnace, dissolved, and then sent to a causticizing section to regenerate sodium hydroxide by treating with lime. The clarified liquor from the causticizing section, containing mainly sodium hydroxide, sodium sulfide and small quantities of unconverted sodium carbonate, is called "white liquor" and is recycled back as the cooking liquor to the digester. One of the major reasons for the success of the kraft pulping process is the established, efficient chemical recovery process whereby more than 95% of the chemicals used in the cooking process can be recovered. Uniformity of composition of the regenerated cooking liquor is a key factor in the production of uniform quality pulp and therefore in the economy of operation of the entire process. The purpose of this research project is to study the causticizing process and develop a reliable kinetic model for the system, with the ultimate goal to improve the chemical recovery operation in the kraft pulping process. A semi-batch reactor already designed and fabricated, capable of operating at higher pressures and equipped to monitor the composition of the main reaction products continuously will be used. A novel in-situ and on-line sampling and analysis system will be used to measure the feed green liquor concentration. Experimental equilibrium data will be supplemented by theoretical calculations. A tubular flow reactor equipped with analyzers to test the green liquor and reaction products will be designed. Such a reactor will also allow necessary adjustment for lime addition and result in uniform white liquor feed to the digester which in turn will allow stable digester operation, improved pulp quality and higher productivity.
