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 evaluatedfrom a technical and economic point of view. Both evaporation of black liquor only and combinedevaporation 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 steamand that an additional 8 m3 of wastewater/ADMT (air dry metric ton) can be evaporated without increasingthe mill\ub4s total live steam demand. They also indicate that the demand for LP steam in the rest ofthe 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 thesaved fuel is rather high in comparison to the electricity price, the total cost for the novel designs will have alower total cost than the traditional designs