Effect Of Additives On Coke Formation And Its Mechanism During Residue Thermal Reaction

In order to improve the coking and fouling condition in the reactor during residue upgrading, the effects of carbonous particles and soluble additives on coke formation and reactor wall fouling were investigated in an autoclave;The mechanisms of additives'effects were elucidated through the characterization of carbonous particle surface, the reaction of model compounds, X-ray photoelectron spectroscopy (XPS) analysis, UV-Vis spectroscopy analysis, FT-IR analysis and the measurement of electric conductivity and Zeta potential.Carbonous particles depressed the coke formation at the initial period of thermal reaction. The shape of the coke was changed, and its size was decreased as well. It was found that the coke was absorbed on the surface of carbonous particles and could be suspended in the oil, resulting in the reduction of the amount of coke deposited on the bottom and wall of the reactor. The particles absorbed the asphaltene selectively in the residue and anchored it on its surface, restraining the aggregation and condensation of asphaltene during thermal reaction. The particles could promote the hydrogen transfer from the donor in aromatics to asphaltene in thermal reaction, and annihilated the radical generated from asphaltene thermal cracking, terminating the condensation of asphaltenic radicals.The soluble additives which could disperse asphaltene depressed the coke formation at the initial period of residue thermal reaction, producing more but smaller coking centers. Subsequently, the coke size was decreased because the coking center was more compared with the same coke yield. Interestingly, the suspending ability of the coke particles was improved. Therefore, the coke content deposited on the reactor bottom was reduced.It was found that the nitrogens at the surface of asphaltene existed in the form of pyrrole and pyridine, that the oxygen existed in the form of carbon-oxygen single bond and carbon-oxygen double bond, and that the sulfurs existed in the form of thiophen and aliphatic sulfide. The head group of soluble additives formed hydrogen bonds with the hydrogens in the hydroxyl and amine groups at the asphaltene surface. Thus, the additives must be absorbed on the asphaltene surface. The acid-base interaction between the additive head group and asphaltene surface should be strong enough and the length of additive side chains should be appropriate for dispersing the asphaltene particles.The measurement of viscosity showed that the soluble additives could suppress the aggregation of the asphaltene colloidal particles by surface adsorption. The size conformed the adsorption of soluble additives on the asphaltene surface. Because the aliphatic side chains interacted with the solvent, the solvation of asphaltene particles also increased. The additives could increase the polydispersity of asphaltene colloidal particles as well.The conductivity of the asphaltene solution also conformed that the soluble additives could interact with asphaltene and improve the colloidal stability of residue. However, after thermal reaction, the additives decomposed and they could not changed the stability of residue any longer.The Zeta potential measurement showed that the main stabilizing effect arose from the side chains, instead of the head groups. Different asphaltenes had different Zeta potentials in the same solvent. The nature of the surface potential might be an important factor to account for the interaction between asphaltene and soluble additives.