Reaction Fronts: How Fast Do They Move and How Do They Affect Large Scale Devolatilization?

Abstract:

Reaction fronts are surfaces separating relatively unreacted from reacted material; the speed they migrate will sometimes control overall metamorphic reaction rate. For dehydration reactions their behaviour is controlled by small scale reaction rate coupled to evolving pressure and porosity. I derive a “steady state” solution for migration speed in which the profiles of fluid pressure and porosity do not evolve in shape, but move at a fixed speed, assuming a rigid porous matrix and incompressible fluid. I assume an externally controlled fluid pressure on one side of the front (in reality in a fracture for example) which controls overall disequilibrium. Reaction front speed is then faster (in a predictable way) for greater disequilibrium, but is also sensitive to initial porosity. There are 2 length (time) scales – a short one over which fluid pressure drops to near the external pressure then, after that, a longer one for reaction to complete. This analysis is useful because it gives a quantitative overview of what controls reaction front speed. More detailed non-steady state numerical models have also been constructed but it is less easy to appreciate why the models evolve in the way they do. The overall reaction rate in a fractured rock mass will be influenced by fracture spacing and local reaction rate, depending on the reaction front speed.