A tool for simulating energy-efficient evaporation using excess heat in kraft pulp mills

Paper i proceeding, 2007

In this paper, we describe and illustrate the use of our simulation tool OptiVap, which has been used successfully in our research group in several papers. OptiVap uses Excel and the built-in Solver to find energy-efficient evaporation plants. The outputs from OptiVap are, for example, the live steam demand, the required heat transfer area and the investment cost. In OptiVap, we can also simulate the consequences of integrating other process units with the evaporation plant. We can e.g. simulate the energy savings of using excess heat in the evaporation plant (process-integrated evaporation) and the evaporation consequences of separating lignin from the black liquor. To illustrate the use of OptiVap, we show that different strategies can be used to optimise evaporation plants. The strategies are symbolised by three objective functions: 1) minimise the steam demand, 2) minimise the heat transfer area and 3) maximise the net annual profit. The simulations are performed for evaporator trains with 6-8 effects, which make use of excess heat from the mill to decrease the live steam demand. The resulting evaporation plants have practically the same profitability, which means that any of the three objective functions can be used to find suitable evaporation plants. Moreover, the first two objective functions do not require any knowledge of the final energy system which brings us to the conclusion that these objective functions are most practical and should thus be used in most cases.