1999 Volume 53 Issue 4 Pages 501-512
In the ambition to force the kraft cook to low kappa numbers without losing pulp quality or yield, many studies have been carried out and a scheme for modified kraft cooking was developed. The basic conclusions are :
1. The alkali profile throughout the cook should be leveled out relative to conventional practice.
2. The hydrogen sulfide ion concentration should be as high as possible in the initial phase and at the beginning of the bulk delignification phase.
3. The concentration of dissolved lignin and sodium ions in the cooking liquor should be kept as low as possible, especially in the final phase of pulping.
4. The temperature should be low in general, especially in the beginning and in the end of the cook.
The principles of modified cooking have been applied to continuous digesters by using split white liquor additions for alkali profiling and by using counter-current cooking methods to minimize lignin concentrations at the end of cook. The computer model is based on the mass and energy flow in the digester. It incorporates delignification kinetics, a penetration model of cooking liquor into wood chips, a chip compaction model in a digester and cellulose degradation kinetics. It can simulate several types of Kamyr continuous digesters : conventional, Lo-Solids, MCC, EMCC and ITC. It can estimate process variables, and predict pulp yield and viscosity at a given kappa number and at a given pulp production. The model simulates a steady-state for process optimization, retrofit feasibility study and education of staffs and operators. This paper describes Lo-Solids cooking of softwood chips and compares the strength and pulp yield with those of conventional kraft cooking.
