Coprocessing: Elemental and molecular weight distributions in unconverted vacuum residues

Abstract

The difference between molecules that are converted to distillates during coprocessing (Cold Lake vacuum bottoms and Forestburg subbituminous coal) and those that are not has been studied. The following procedure was used. The fraction of the liquid product boiling above 525 °C was separated into five subfractions by preparative scale gel permeation chromatography (GPC). Each subfraction was weighed. Then the following measurements were made on each subfraction ; molecular weight, elemental analyses (C, H, N, S, V, and Ni), and carbon-13 nuclear magnetic resonance. Five significant observations were made. (1) The molecules that were converted had larger H/C atomic ratios and smaller N/C atomic ratios than the feedstock molecules. (2a) S, V, and Ni heteroatoms could be removed without causing much change in molecular weight. (2b) It was not possible to increase the H/C atomic ratio or decrease the N/C atomic ratio without decreasing the molecular weight. (3) Generally, the unconverted +525 °C residue molecules became smaller as the processing severity increased. (4) The unconverted molecules retained their side chains at mild processing severities (425 °C). When the processing severity increased (450 °C), side chain carbon began to be removed. (5) If there was insufficient reaction time to provide enough hydrogen, then molecules which were larger than the feedstock molecules were formed. Feedstock molecules that lost their hydrogen rich fragments (by cracking or by hydrogen transfer) without contacting enough hydrogen to remove their nitrogen or metal heteroatoms may have oligomerized. If the reaction time was increased to allow more contact time with hydrogen, the oligomerized molecules which were larger than the feedstock molecules disintegrated. Finally, conversion of residue molecules to distillate molecules appeared to be limited by hydrogen addition. For conversion, hydrogen was required either to hydrogenate aromatic rings or to remove nitrogen heteroatoms. There are other important requirements for hydrogen which are not primary steps in the conversion of large molecules to small ones. They include capping pyrolysis fragments and the removal of other heteroatoms (sulfur, metals).