Simulations comparing conventional evaporation plants with plants using excess heat

Pulp and paper mills are large energy consumers which can often achieve economic savings by implementing energy-saving measures. The process unit with the greatest energy demand in a mill is usually the evaporation plant. If excess heat can be made available in the mill, and the heat can be used in the evaporation plant, significant energy-savings can be achieved. In this paper, this kind of energy-efficient evaporation is called process-integrated (PI) evaporation, and the paper investigates the techno-economic consequences of PI evaporation. Theoretical plants with 6–8 evaporation effects are simulated using an in-house simulation tool called OptiVap. Conventional plants are used as reference, and evaporation plants with either lower surface condenser temperature or extraction of lignin are included. The results show that the additional profit of PI evaporation plants is 0.3–1.5 €/ADt in comparison with conventional plants. By lowering the temperature of the surface condenser, the profit is raised by 0.6–0.9 €/ADt for both conventional and PI plants. With lignin extraction, the PI plants are 0.7–1.7 €/ADt more profitable than the conventional ones