Kinetics of supercritical water oxidation of isopropanol as an auxiliary fuel and co-fuel

Abstract

Nowadays the studies that deal with the supercritical water oxidation (SCWO) of problematic wastes are focused in the search of practical solutions for the SCWO main drawbacks: corrosion, plugging, and high running costs. Regarding the high running costs, an important fact is that significant energetic requirement does exist during the process start-up, as well as to reach an autothermal operation if the waste to be treated does not have enough inherent heating value. The use of organic solvents [such as methanol (MeOH), ethanol (EtOH) or isopropanol (IPA)] as auxiliary fuel (to aid running the start-up stage)/co-fuel (to improve the steady-state stage) in the SCWO of certain wastes is emerging as a possible solution to help overcoming these problems. Many works in the literature use IPA as fuel in the SCWO process. However, there are very few laboratory studies that detail, from a practical point of view, the kinetics of the IPA oxidation in supercritical water (SCW) with the aim of unveiling the insights of its behaviour as an auxiliary fuel/co-fuel in the SCWO process and the development of further works on the simulation at a larger scale.The aim of this work was to develop a detailed and practical set of kinetic equations easily applicable to further studies on simulation and experimentation at a larger scale. In addition, pyrolysis–hydrolysis tests in SCW were also conducted concluding that, in the case of IPA, these reactions do not significantly interfere with SCWO. The required experimentation was performed in a tubular reactor system at laboratory scale at a constant pressure of 25MPa, using pure oxygen as oxidant and monitoring the efficiency of the oxidation process in terms of reduction in chemical oxygen demand (COD) and total organic carbon (TOC) versus the residence time. This study consists of two major blocks: (a) a set of experiments on SCWO of IPA under a constant excess of oxygen (=100%) and different temperatures ranging from 673 to 773K. Two-parameter mathematical models involving two steps (a fast reaction followed by a slow reaction) were proposed to describe the IPA SCWO kinetics and to calculate the corresponding kinetic parameters, assuming a zero order for oxygen concentration; and (b) a complementary set of experiments on SCWO of IPA at a constant temperature (748K) and different oxidant coefficients from n=0.5 (50% of stoichiometric oxygen) to n=2.0 (100% oxygen excess), proposing a kinetic model for oxygen concentration dependence whose parameters were determined using a Runge–Kutta-fourth-order algorithm. The kinetic models proposed allow the prediction of the COD or TOC conversion with or without respect to oxygen supply in order to optimize the SCWO operating conditions and to minimize investment and operating costs.