Free radicals in coal and coal conversions. 7. An in-depth experimental investigation and statistical correlative model of the effects of residence time, temperature and solvents

Abstract

Previous papers in this series have described the methodology for in-situ studies and the relative effect of various process variables on the concentration of free radicals from a Powhatan No. 5 coal. This paper describes the role of temperature and solvents for the same coal. In addition, a correlative mathematical model has been developed that fits the data and is used to examine the sensitivity of free radical formation to the various process variables. E.s.r. measurements have been made at various temperatures from 400–480 °C. The solvents that have been used are tetralin, naphthalene and SRC-II heavy distillate. The main conclusions from this work are: 1. (1) in general, the effectiveness in quenching the coal free radicals increases in the order: naphthalene, SRC-II heavy distillate, tetralin; 2. (2) the free radical concentration of the liquefaction slurry generally increases with increasing temperature; and 3. (3) the rate of quenching of free radicals by SRC-II heavy distillate is closer to that of tetralin at lower temperatures and approaches that of naphthalene at the higher temperature. From the results of the full correlative model, it has been found that changes in temperature, nature of solvent and residence time, and their interactions, account for approx. 90% of the changes noted in the free radical concentration. The spin concentration is dependent upon reaction time, and this dependence is also influenced by solvent and temperature. The activation energies for quenching the free radicals are distinctly different for the different solvents.