Evaporation of black liquor and wastewater using medium-pressure steam: Simulation and economic evaluation of novel designs

In this paper, novel evaporation plant designs that use medium-pressure steam and deliver low-pressure (LP) steam to the steam network as well as conventionally designed plants are simulated and evaluated from a technical and economic point of view. Both evaporation of black liquor only and combined evaporation of black liquor and wastewater are analyzed. The results show that the novel designs suggested in this work all have large potential to save live steam and that an additional 8 m(3) of wastewater/ADMT (air dry metric ton) can be evaporated without increasing the mill's total live steam demand. They also indicate that the demand for LP steam in the rest of the process is less important for the savings in live steam. The total cost for the novel designs and the relationship between that cost and the cost for conventional designs depend on the economic conditions assumed. If the investment in a more energy efficient evaporation plant is considered from a strategic point of view using an annuity factor of 0.1 and if the cost of the saved. fuel is rather high in comparison to the electricity price, the total cost for the novel designs will have a lower total cost than the traditional designs. (C) 2002 Published by Elsevier Science Ltd.