Mass Transfer and Kinetics

Herbert Sixta

Oxygen delignification of pulp is a three-phase reaction system consisting of an

aqueous phase, suspended pulp fibers, and the oxygen gas phase (oxygen must be

transferred from the gas to the liquid phase and then from the liquid to the solid

phase). As a first step, oxygen dissolves in the aqueous phase and is then transported

through the liquid to the liquid–pulp fiber interface. The dissolved oxygen

subsequently diffuses into the fiber wall and then reacts with the wood components,

preferably with the residual lignin structures.

The full description of the oxygen delignification process requires the following

information:

_ The solubility of oxygen in the alkaline solution.

_ The oxygen mass transfer rate in the aqueous phase.

_ The effective diffusion coefficient of oxygen inside the fiber wall.

_ Stoichiometry and chemical kinetics of the oxygen delignification

reactions.

The physical transport of oxygen gas through the immobile aqueous film layer by

diffusion is the rate-determining step for oxygen delignification. Therefore, fluidization

of the pulp suspension is regarded as a prerequisite for oxygen delignification.

It is generally agreed that the course of both oxygen delignification and carbohydrate

degradation is mainly affected by the three primary process variables, temperature,

sodium hydroxide concentration, and dissolved oxygen concentration.

Furthermore, the ionic strength is also thought to influence the delignification

rate. In contrast to kraft pulping, an increase in ionic strength during oxygen

delignification was reported to accelerate the delignification rate [1]. Olm and

Teder explained this observation by assuming that the rate-controlling reaction

occurs between two negatively or two positively charged species.

The prerequisite of kinetic investigations is to avoid any mass-transfer limitations.

The influence of pulp consistency on the rate of delignification has been

ascribed to insufficient mixing in both the low and high consistency ranges [2,3].

Argarwal et al. reported that no significant effect of consistency is observed for

oxygen delignification of mixed southern hardwood in the range 0.5 to 12%, provided

that there is sufficient mixing. Nevertheless, kinetic investigations are preferably

carried out at ultra-low consistencies (0.3–0.5%) in a well-mixed batch

reactor to ensure constant concentrations of sodium hydroxide and dissolved oxygen

(further information about mass transfer aspects are provided in Section

4.2.3).

7.3 Oxygen Delignification 671

Дата добавления: 2015-10-21