ARTICLE pubs.acs.org/EF
Influence of Depressurization and Cooling on the Formation and Development of Mesophase S. Reza Bagheri, Murray R. Gray, and William C. McCaffrey* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB Canada T6G 2V4 ABSTRACT: The effect of cooling and depressurization on the formation and growth of carbonaceous mesophase in petroleum vacuum residue was investigated using a novel stirred hot-stage reactor. This apparatus allowed the in situ observation of mesophase formation at 440 °C and 46.5 MPa under both hydrogen and nitrogen atmospheres. The use of a magnetic stirrer enabled experiments with milliliter volumes of liquid and the addition of catalyst. The in situ observations showed that cooling at constant pressure did not change the amount and size of the mesophase domains, except to arrest their growth by stopping the chemical reactions. Depressurization to atmospheric pressure, while maintaining reaction temperature, after the onset of mesophase formation increased the amount of observable mesophase material significantly. Depressurization before the onset of mesophase formation also induced the formation of observable mesophase earlier than if the reactor had been maintained at pressure.
’ INTRODUCTION Coke formation is a major problem in the petroleum industry because of its effect on liquid yield and catalyst deactivation. In addition, it can foul reactor internals and downstream vessels and force shutdowns. These deposits are insoluble in the liquid phase at process conditions, but the most common definition of coke in laboratory studies is material that is insoluble in toluene.1 Of particular interest is the accumulation of such material as a foulant inside the process equipment. Brooks and Taylor2 first observed that, during the thermal cracking of petroleum and coal tar pitches, an intermediate phase is formed, which is anisotropic. This intermediate phase, known as carbonaceous mesophase, is a discotic nematic liquid crystal state,3 while pitch is a convenient term for heavy liquid fractions that have been significantly altered by thermal cracking reactions. Carbonaceous mesophase appears during the heat treatment of pitch in the temperature range 350500 °C, as optically anisotropic spheres surrounded by an isotropic liquid matrix. Mesophase is thought to consist of clusters of approximately planar aromatic molecules with significant orientational order but no long-range positional order.4 The onset of mesophase is of particular interest in hydroconversion processes, because this phase can give severe fouling of the reactor internals due as a result of coalescence and adhesion.5,6 The formation of new phases, such as sediment or mesophase, can limit the conversion of the vacuum residue.7 Formation of a new anisotropic phase requires a combination of chemical reaction, nucleation, and diffusion of material from the surrounding liquid to the new phase. Greinke et al.8 studied the composition of coexisting mesophase and isotropic phase during the thermal cracking of petroleum pitch at 400 °C with time. They suggested that the initial nucleation of mesophase was driven by the removal of low-boiling components (
