Energy-Efficient Closed-Cycle Bleaching, 1997 Minimum Effluent Mills Symposium Proceedings

The overall energy consequences have been studied when closing the water system in a kraft pulp mill, producing TCF pulp. Results show that the net energy requirements for closed and open systems in principle are similar, provided that efficient process integration is applied. However, the need for heat exchangers is much greater in an open system. A system with a high degree of closure is often automatically process-integrated regarding energy aspects, but can in many cases be further optimized. Pinch analysis, where the different heat sources in the mill are considered, shows a potential to pre-evaporate 2-6 m 3 /ADt utilizing excess heat from the surrounding process. This potential is reduced if the bleaching sequence includes a low-temperature stage or if the mill already has exploited suitable heat sources in an efficient way. Striving towards the optimal energy solution, the choice of bleach plant design is in some cases important. Wash presses allow a higher degree of energy recovery compared to wash filters. The choice between counter-current or split-flow washing is not critical, according to our results. However, it will affect the demand for heat exchanger capacity, in favor of the former method.