Fluid Dynamics & Aerodynamics

Biography

Biography: I V Sharikov

Abstract

Hydrothermal synthesis is widely used for the production of various nanodispersed oxide materials. Reactions under hydrothermal conditions are complicated and usually they are accompanied with heat generation or heat absorption. Heat flux calorimetry is a powerful instrument for kinetic study and developing mathematical models of hydrothermal reactions. A mathematical model makes it possible to determine optimal experimental conditions for the production of a definite material on the base of a limited number of kinetic calorimetric runs. But in order to apply the kinetic data to reactors of larger volume one should take into account heat transfer, mass transfer phenomena and non-uniform temperature distribution in a definite apparatus at the chosen initial conditions and in course of hydrothermal synthesis. Reaction vessel of C80 Calvet calorimeter (SETARAM Instrumentation) is a micro-autoclave of 8.5 cm³ volume without mechanical stirring. Heat transfer and mass transfer inside it are run due to natural convection while heating to a chosen temperature of an isothermal run. And temperature gradient in this case is rather moderate (yet not negligible) as the reactor is relatively small. If we pass to the reactor of a larger volume (e.g., 1 liter) – we find that the real temperature mode in it is far from that in a kinetic vessel at the same initial conditions. In order to take into account the temperature and conversion distribution due to natural convection in course of a definite hydrothermal synthesis we have developed a mathematical model that takes into account convection inside a hydrothermal reactor together with the chemical reaction. Convective flows were described at the base of Business approach and the differential equations system was solved with applying Convex program package that takes into account size and geometry of the reactor, reaction mixture properties, heat transfer peculiarities inside and outside and heat generation due to chemical reaction. It was found that temperature and conversion distributions in the calorimetric vessel and in the 1 liter reactor were rather different at similar initial conditions from the very beginning. Time of reaching the stationary temperature profile in the bigger vessel at implementing, e.g., isothermal mode is comparable with total duration of the run, and stationary temperature gradient is bigger as well. This indicates of the necessity to estimate rigorously the natural convection and heat transfer phenomena at scaling the hydrothermal synthesis for the reactor of bigger volume without mixing. Kinetic models developed on the base of calorimetric data cannot be directly applied to simulating the hydrothermal synthesis process in such a reactor.