Pulping reactions diffusion

The grafting reaction depends upon the degree of substitution as well as the kind of pulp used. Introducing acetyl groups in the cellulose chains (high substitution) causes a large reduction of its swellability, which reduces the diffusion of the reactants. Thus, acetylation lowers the graftability of the cellulose. 

In the acidic chlorination stage of pulp bleaching, most of the chlorine is consumed rapidly within 5 to 10 minutes. This initial phase is diffusion controlled, and effective mixing of the pulp slurry facilitates the reaction. 

As previous chapters have shown, a number of materials cause adverse reactions to the cells of the pulp, specifically materials that release the monomer HEM A [2]. This monomer has been shown to be capable of diffusing through thin layers of dentine and entering the pulp [3]. Once within the pulp, it proves to be cytotoxic and causes the death of otherwise viable cells within the varions parts of the pulp [4,5]. In addition, some light-cure units used in dentistry have been shown to emit sufficient heat to raise the temperature within the pulp to levels at which the cells are destroyed [6,7]. 

Biotechnological processes, such as pulping, are best carried out at elevated temperatures. The increase in temperature has a significant influence on the bioavailability and solubility of organic compounds, and is accompanied by a decrease in viscosity and an increase in the diffusion coefficient of organic compounds. Consequently, higher reaction rates due to smaller boundary layers are expected. The bioavailability of insoluble environmental pollutants can also be improved dramatically at elevated temperatures allowing efficient bioremediation (37). 

An electrochemical reactor with anolyte and catholyte divided by a cation-permeable membrane or semipermeable separator is required to preclude the diffusion of sulfide/polysulfide anions from anolyte to catholyte and to avoid reduction of polysulfide formed at the anode by reaction (9), being reduced at the cathode by the reverse reaction. Similar processes have been reported [18] for producing polysulfide (and hydroxide) solutions at current densities of 5 kA m for Kraft paper pulping processes. 

In a fermentor undergoing carbonic maceration, the grape berry is transformed by anaerobic metabolism reactions of its own cells. These reactions are independent of any yeast involvement and have been covered in the preceding section. Tissue degradation favors the maceration phenomena involved. Phenolic compounds, anthocyanins, nitrogen compounds and other components of the solid parts of the berry are diffused in the juice of the pulp. 

Mixing and reaction time constants are not readily calculated for pulp suspensions. Mixing rate is influenced by the complex suspension rheology. Bleaching rate is influenced by chemical diffusion into the fiber wall and can be controlled by mass transfer. Further, in laboratory experiments a net reaction rate is measured and may have been influenced by the mixing conditions during the test. Despite these concerns, a number of estimates can be made. 

The time for diffusion (without reaction) to increase the average concentration within a fiber wall to one-half of its ultimate value, with the fiber treated as a fully collapsed slab of thickness equal to two fiber wall widths, gives a 0.19. For a fiber wall thickness of a = 4.0 x 10 m and an effective diffusion coefficient of D = 1.3 X 10 m /s, Xd = 0.0023 s. The ozonation of pulp fibers follows a shrinking core mechanism, limited by the rate of ozone diffusion through the fiber wall. Exposed fibers are completely penetrated and reacted in seconds under typical operating conditions (Bennington et al., 1999). 

S.Oi Diffusion in liquid Reductive reaction Pulp and paper... 

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