Influence of pressure variations on the formation and development of mesophase in a petroleum residue

Abstract

The objective of this study is to search for a method, involving control over the pressures within a pyrolyzing system, to produce non-coalesced mesophase spheres of required sizes. Semicokes were studied from pyrolyses of a petroleum residue, R1, carried out in a tube reactor, 440–445°C, 1.0 MPa pressure, and soak times 1.0–2.0 h. In some pyrolyses, pressures were reduced from 1.0 to 0.1 MPa and the pyrolysis continued at the same reaction temperature at 0.1 MPa. These experiments are termed depressurizations. Semicokes were examined by optical microscopy to measure mesophase content as well as distributions of size of mesophase spherules and sphere concentration (number of spheres mm−2). High-pressure (1.0 MPa) pyrolyses favour the coalescence of mesophase spheres to form bulk mesophase. Initially formed spheres do not grow beyond a certain size indicating that rates of coalescence exceed rates of growth of the spheres. Depressurization causes a loss of volatile matter with a proposed resultant increase in viscosity of the remaining isotropic phase. Some volatile material may also be removed from the mesophase spheres themselves, again promoting a viscosity increase. As a result of this, the spheres increase their diameter and there is less production of bulk mesophase. With depressurization, coalescence is not stopped entirely but is reduced considerably in extent.