Effect of Brownstock Washing and Carryover on the Function of the Oxygen Delignification Process, TAPPICon26

Washing after cooking affects how much dissolved substance is transferred to the oxygen stage. Carryover has A great impact on the function of oxygen (O2) delignification. It is formed from the wash loss of the cooking-derived dissolved substances (raw black liquor, RBL) and dissolved substances which has gone through the oxygen delignification process (Oxidized black liquor, OBL). With carryover come substances which consume oxygen. On the other hand, it also brings alkalis, which speed up delignification. Recently, the model has been used to evaluate and optimize the operation of oxygen delignification processes. To refine this model, more information is needed on the nature and effect of carryover on the performance of oxygen delignification processes.

This work compiles results and data from ten Scandinavian pulp mills. Measurement of total dissolved solids (TDS) using a process refractometer was used as the carryover measurement unit, and chemical oxygen demand (COD) and conductivity were also measured from some samples.

Results over a period of 20 years show that the washing operations and washing emphasis vary widely between mills. The results showed that cooking-related carryover affects the operation of the oxygen stage by consuming oxygen and heating the process. In the studied hardwood process, 100 kg COD/ADt in the carryover increased the temperature in the reactor by 1.1 degrees and oxygen consumption by 0.7 kg/ADt. The RBL oxygen consumption varied in different laboratory studies from 0.1 to 0.3 kg O2/ kg TDS. In this mill study consumption of less than 0.13 kg O2/ kg RBL was obtained. The values published in this work can be utilized in refining the oxygen stage model, especially when assessing the effect of carryover on oxygen consumption.

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