AN ENERGY-EFFICIENT EVAPORATION PROCESS FOR TREATING BLEACH PLANT EFFLUENTS

Simulation results of an energy efficient evaporation process are repo rted for the treatment of bleach effluents in the pulp and paper indus try. Due to the low concentration of the effluent stream, the evaporat ion process must have a high degree of energy efficiency in order to c ompete with other treatment alternatives, such as ultrafiltration, ads orption, ion exchange and biological treatment. For a pulp and paper m ill with an annual capacity of 335,000 ton of bleached kraft pulp, the capacity of the spent black liquor evaporation plant is about 7.5 ton water/ton pulp if its concentration is increased from 16 to about 65 wt% dry substance. An evaporation plant for the bleach effluent was si mulated for a flow rate corresponding to 7.2 m(3)/ton pulp and a conce ntration of 1.4 wt% dry substance. This stream is to be concentrated t o 16 wt% resulting in an evaporation capacity of 245 ton water/h. The total evaporation capacity for both evaporation plants would be increa sed by 91%. Optimal energy conservation strategies were investigated w here an absorption heat transformer unit (AHT) is integrated with both the existing black liquor evaporation plant and the proposed bleach e ffluent evaporation process. Different process configurations were sim ulated using a flow sheeting program, developed for simulation of gene ral multiple-effect evaporation processes and absorption heat pump sys tems. Using real operating data From a major Swedish pulp and paper mi ll, simulation results are reported for the optimum location of the AH T unit. Consequently, the energy requirements for both evaporation pla nts would increase by only 6.6%. One of the main practical features of this process is that integrating the AHT unit would require minimum c hanges in the existing plant. An economic analysis resulted in a cost of 23 SEK/ton of pulp for the first alternative where the pre-concentr ated 16 wt% effluent stream is concentrated further to 65 wt%, within the existing black liquor evaporation plant, and then recycled to the recovery boiler. The corresponding cost of 49 SEK/ton of pulp would re sult for the second alternative, where the 16 wt% effluent stream is s prayed on the waste from bark peeling and co-burned in the bark boiler .