Optimal recycle/reuse policies for minimizing the wastes of pulp and paper plants∗

Abstract The purpose of this work is to develop a systematic and generally applicable methodology for the optimal design of recycle/reuse process networks for reducing the emission of hydrogen sulfide from pulp and paper plants. The problem is formulated as an optimal synthesis task with the objective of minimizing the total annualized cost of the processes used for the waste minimization task while satisfying all environmental requirements. A systematic procedure is developed in which tradeoffs between cost and waste reduction are established. A key tool for this procedure is the recently developed technique of synthesizing mass‐exchange networks. This approach allows for the simultaneous screening of all potential hydrogen sulfide recovery technologies. Such separation technologies includes those employing internal (process) resources as well as external resources. Internal resources correspond to mass‐separating agents that are available in a pulp‐and‐paper plant which can be used for the removal/recovery of hydrogen sulfide from the waste streams. On the other hand, external resources refer to streams that are not involved in the pulp‐and‐paper process but may be purchased for use in recovering hydrogen sulfide from air emission streams. A case study is solved to demonstrate the applicability of the proposed approach. The results of this case study clearly indicate the economic advantage of using white liquor for hydrogen sulfide removal in emission streams compared to alternative removal technologies and destruction (incineration) techniques.