Second-law analysis applied to a kraft pulp mill

One important step towards a sustainable process is to maximise the thermodynamic efficiency. When this efficiency is investigated, it is of crucial importance to have an objective method. In this work, one such method, a second-law analysis based on the entropy conception, is presented for a Swedish kraft pulp mill. The study was limited to the sodium and calcium cycles, i.e. the recovery cycle and the paper pulp digester. In order to perform a second-law analysis, reliable material and heat balances for the system are needed. Data for the calculations of these balances were: flows, temperatures, dry contents and ion contents (the ions analysed were sodium, potassium, hydroxide, sulphide and carbonate). The results from the second-law analysis show that there is a small difference between the heat balance and the entropy balance when the least favourable equipment is determined, i.e. the same equipment has the highest heat loss and the highest increase of entropy. The two balances differ, however, in the relative distance between different equipment.