Modelling continuous adsorption processes

Fig. 8.17 Complex plane plots of complex capacitance, C, defined using Eq. (8.28) for limiting cases of slow continuous line), fast (diffusion limited, dotted line), and intermediate (dashed line) adsorption. Insert equivalent electrical model for this process (From Ref. [367], copyright (2002), with permission from Elsevier)...

Two kinetic models, namely, pseudo-first-order and pseudo-second-order, were used to investigate the adsorption process of methyl orange, methyl blue and safranine T onto synthesised and commercial zeolite. Kinetic parameters along with correlation coefficient for the pseudo-second-order kinetic model are listed in Table A.3. The calculated correlation coefficient is closer to unity for the pseudo-second-order kinetic model than the pseudo-first-order kinetic model. Therefore, the sorption reaction can be approximated more favourably by the pseudo-second-order kinetic model for methyl orange, methyl blue and safranine T onto synthesised and commercial zeolite. MPSD error function values as shown in Table A.3 are also considerably lower for the pseudo-second-order kinetic model, reinforcing the applicability of the pseudo-second-order kinetic model. It may be seen that the initial sorption rate (h) continuously increased with increase in Cq. This is due to the increase in driving force due to the increase in Q. 

Reactor control models for monoliths require a more detailed study of the time scales of all occurring subprocesses, because of their dynamic character. Under dynamic circumstances, the rates of the individual elementary steps of a catalytic cycle, such as adsorption, surface reaction, and desorption, are not equal to each other anymore, since the time scales of the corresponding processes may differ by many orders of magnitude. Therefore, accumulation effects on the catalyst surface have to be taken into account as well, which demands that continuity equations for surface species be included in the model. Such aspects may even play a role in the steady state if the kinetics depend on rate-determining steps that change according to the concentration level of the reactants... 

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