Optimization of the usage of oxygen in the O2 delignification process, TAPPICon26

In the oxygen delignification process the target is to achieve a high concentration of dissolved oxygen in the pulp suspension. This is obtained by charging enough gaseous oxygen and dispersing it into small bubbles so that mass transfer is adequate to keep the reactor saturated with oxygen. Then there arises the question of how small the oxygen bubbles should be, and how this can be affected. Another question is how much of the charged oxygen is consumed in the process. The aim of this presentation is to get answers to these questions by summarizing earlier studies and presenting new results obtained in mill tests. These issues have recently been studied extensively based on a modelling approach where the pulp properties are calculated based on the concentration of dissolved oxygen in the reactor and how much oxygen is consumed by reactions with fibers, dissolved substances and oxidized white liquor.

In processes where the average bubble size is small, around 0,1 mm, it was concluded that mass transfer is very rapid and reactors are saturated with dissolved oxygen. In processes where the bubbles were bigger the bubble size became bimodal having also bubbles around 1 mm or bigger. The surface area of these bubbles per unit volume is about a hundred times smaller compared to the small bubbles of the same total volume and according to mill tests and modelling studies their capability to dissolve oxygen is very poor. Consumption of oxygen was shown to be usually clearly less relative to the charge in different mills. The residual gas bubble and oxygen concentration measurements help to quantify the oxygen consumption in the process and there is also potential to use these measurements for the optimization and adjusting the oxygen charge.

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