Influence of thermoplastic properties on coking pressure generation: Part 3 – Evidence and role of pore coalescence in the mechanism for pressure generation

Abstract

This work follows on from two papers which presented a hypothesis for the mechanism behind oven wall pressure. It was proposed that oven wall pressure results when the period of pore growth is lengthy prior to coalescence and/or the period of coalescence is minor ahead of resolidification. When this occurs a thick swollen band of closed pores in the viscoelastic medium results which exerts pressure on the oven walls. The band is a resistance to volatile transfer and the largely closed pore network remains throughout resolidification further exacerbating pressure on oven walls. This hypothesis has resulted from a mapping of the viscoelastic properties obtained from a rheometer against the plate behaviour of the rheometer where pore nucleation and growth has been characterised by an increase in plate gap (ΔL) when the plate is held under constant axial force (AF) or an increase AF when the plates are held at constant gap. Pore coalescence has been characterised by a peak in AF or a decrease in the rate of expansion of ΔL. Because the behaviour of either ΔL or AF has brought about our understanding of coking pressure this paper attempts to prove that pore coalescence can be monitored from plate behaviour.As such this paper attempts to track pore growth and coalescence using mercury porosimetry, permeability, scanning electron microscopy and optical microscopy, all of which have been carried out on quenched coke samples. Scanning electron microscopy images showed that the ΔL profile provides an accurate account of the onset of bubble nucleation, growth and coalescence. Porosity measurements indicate that the proportion of open pores in the 1–100μm range increases significantly after the peak in AF or decrease in the rate of expansion of ΔL, indicating that these points coincide with a bubble coalescence phenomena. Optical microscopy showed a distinct difference in pore structure between the image corresponding to the peak in axial force and images after, whereby the number of isolated cells decreased and pore walls became thinner. Permeability measurements on quenched samples throughout the softening process show an initial drop in permeability due to the loss of interparticle voidage and the onset of swelling followed by a steady increase coinciding with the peak in axial force and decrease in the rate of ΔL expansion. This work has helped to reinforce the proposed mechanism for coking pressure. Furthermore, permeability measurements of quenched samples at 550°C show that the high pressure coals tend to have the lower permeabilities at this temperature.The finding that pore growth and coalescence behaviour is what impacts on coking pressure has opened the door for finding novel ways to manipulate pore behaviour and therefore manipulate coking pressure. Understanding and manipulating pore growth behaviour has other implications, largely the understanding for coke strength development, which is currently being explored by the authors.