Diffusion of solvents in coals: 1. measurement of diffusion coefficients of pyridine in Elbistan lignite

The aim of the present report is to measure the diffusion coefficients at temperatures about 20-27 degreesC, the activation energies of diffusion, and to determine the type of transport mechanism of diffusion of pyridine in macromolecular structure of Turkish Elbistan lignite (C: 53.0 wt %, dmmf). The raw coal sample was demineralized with HCl and HF by standard methods. The raw and demineralized coal samples were extracted with pyridine. Pyridine uptake of the coal samples was recorded at temperatures 20-27 degreesC in an adiabatic setup until a constant weight was attained. The extent of swelling of original and treated coal samples in pyridine were measured. Pyridine extraction created a fraction of enlarged particle size due to irreversible solvent swelling. The diffusion coefficients were measured from the slope of graphs of M-t/M-infinity versus t(1/2) The diffusion of pyridine in the raw coal seemed to be less, compared to those of the treated coal samples. Extraction of the raw coal with pyridine extended the pyridine diffusion in the coal very little. The formation of new carboxylic acid groups in acid-washed samples enhanced diffusion of pyridine. In all of the samples the diffusion constants increased linearly with an increase in the temperature. The greater diffusion coefficients are encountered with the coal samples which swelled more in pyridine. It seemed that the diffusion coefficients increased with swelling and acid washings as a result of structural variations decreasing the activation energy of diffusion of the solvent to the coal. The transport mechanism of pyridine in the macromolecular coal network of Elbistan lignite was not relaxation controlled. The diffusion of pyridine in the low rank Elbistan lignite obeyed generally Fickian mechanism or an intermediate case of Fickian-anomalous mechanism