Creating a consistent thermodynamic database is the first step required to perform a process integration study. A method based on the use of data identification and reconciliation has been developed to model the mass and the energy balances of a pulp and paper sulfite mill. It combines data from different sources: online measurements, monthly report, specifications sheets and uses the data from both the process and the utility side in order to compute the nominal process states that are used to define the heat transfert requirement of the process.