Absoiption physical

There is no sharp dividing line between pure physical absorption and absorption controlled by the rate of a chemiem reaction. Most cases fall in an intermediate range in which the rate of absoiption is limited both by the resistance to diffusion and by the finite velocity of the reaction. Even in these intermediate cases the equihbria between the various diffusing species involved in the reaction may affect the rate of absorption. 

Hoftyzer and van Ki evelen [100] invesdgated die combination of mass ttansfer togedier widi chemical reactions in polycondensation, and deduced die ratedetermining factors from the description of gas absoiption processes. They proposed dii ee possible cases for polycondensation reactions, i.e. (1) die polyconden-sadon takes place in the bulk of die polymer melt and die volatile compound produced has to be removed by a physical desoiption process, (2) die polycondensation takes place exclusively in the vicinity of die interface at a rate determined by bodi reacdon and diffusion, and (3) the reacdon zone is located close to die interface and mass ttansport of the reactants to diis zone is die rate-determining step. 

Most importantly, however, photoexcitation methods allow the detection of transient species. While an entity with a lifetime of 50 ns or less can be comfortably analyzed by transient absoiption or fluorescence spectrometry, other physical methods of steady-state character are likely to fail. For example, the timescale of most NMR experiments is longer than ca. 100 ms. which renders this method unsuitable for studies of short-lived species. 

The effect of hindered amine promoters on the rate of CO2 absorption into a potassium carbonate solution has been evaluated using a single sphere absorber (Sartori and Savage, 1983) and a pilot scale packed column (Weinberg et al. 1983). The single sphere absorber tests showed that unpromoted potassium carbonate provides a mass transfer enhancement in the range of 10-30 times that of simple physical absoiption. The addition of a conventional amine activator (DEA) further increases the mass transfer rate by a factor of about 3, and the substitution of a hindered amine for the DEA gives an additional increase by a factor of about 2. 

Thus the assumption of the same value for interfacial area in physical and chemical absoiption leads to uncertainty, especially if the mass transfer coefficient is deduced from lq,a measured by physical absorption or desorption and from a in a chemical absorption. The effective interfacial area in the case of the fast reaction systan vfliere the absorbing capacity is increased by a chanical reactant is substantially larger than the effective interfacial area for physical absorption or desorption, as pointed out by Joosten and Danckwerts (70) that introduced a correction factor Y> the ratio between the increase of liquid absorption capacity (l+Cgo/zCj ) and the increase of mass transfer due to chanical reaction (E). 

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