Synthesis of heat recovery systems in paper machines with varying design parameters

The heat recovery system (HRS) is a vital part in a paper machine when it comes to the overall energy economy in papermaking. For a typical newsprint machine more than 60% of the exhaust energy from the dryer section can be recovered, resulting in a recovery of about 30 MW. The synthesis task of a HRS is a decision process where the target is on the one hand to achieve maximal energy recovery and on the other hand to obtain this recovery with minimal investment costs. These goals are contradictory and thus the problem is to find a solution minimizing the overall costs, considering simultaneously both energy and investment costs. This synthesis task can be performed with e.g. pinch-analyses or optimization methods. One of the first tasks for the designer is to decide which design parameters, including process flow streams, temperatures and heat transfer coefficients are to be applied. This task is in general not trivial and the result will have a great impact on the overall economy of the final HRS. One challenge is how to take into account uncertainties and known variations in some parameters. The desired design must be capable of handling all evolving situations but it should also be the most economical one when considering the duration of the different operational situations. In this work the importance of taking the variations and uncertainties into account in the design stage is shown with a case study.