フルオレセインの熱分解挙動に関する一次反応速度式による解析

Abstract

Sodium fluorescein (uranine) has been used frequently in tracer tests that carried out in order to monitor recirculating fluids utilized for heat exchange with artificial geothermal reservoir such as hot dry rock geothermal system. It is known that fluorescein decomposes in high temperature condition. In the case which tracer solution contained fluorescein is diluted to around 0.1 mg/l of fluorescein concentration by mixing with existing natural geothermal brine in the underground, fluorescein may be strongly affected by thermal effect. In spite of being relatively short residence time, the recirculating fluid utilized in hot dry rock geothermal system is used under the condition in which the temperature dramatically changes. Therefore it is important to understand the detailed thermal decomposition characteristic of fluorescein.In this study, the experiments for thermal decomposition of fluorescein were carried out using flow-type reactor, and the effects of the initial concentration of fluorescein and temperature on the thermal decomposition behavior of fluorescein were examined experimentally and kinetically. It was found that the proportion of decomposed fluorescein to the starting became higher as the initial fluorescein concentration became lower. The kinetics of the thermal decomposition of fluorescein was approximately regarded as first-order from the results in our experiments. On the other hand, the rate equation for thermal decomposition of fluorescein should contain the term of the dissolved oxygen concentration, because the oxidation reaction is greatly related to the decomposition of fluorescein. Therefore, the effect of the dissolved oxygen concentration on thermal decomposition of fluorescein was introduced as the term of the abundance ratio of dissolved oxygen for fluorescein in the rate equation. That also means that the influence of the initial fluorescein concentration on the thermal decomposition of fluorescein is represented as the initial abundance ratio of fluorescein for dissolved oxygen. Then the new kinetics on thermal decomposition of fluorescein was suggested considering the initial concentrations of fluorescein and dissolved oxygen.