Research On The Colloidal Stability And Hydrogen-Transfer Of Vacuum Residue During Thermal Conversion

The colloidal stability and hydrogen-transfer ability of vacuum residue during thermal conversion are main factors affecting the coke formation and suppression. In order to enhance liquid yield and suppress coke yield during vacuum residue thermal conversion, the colloid chemistry and hydrogen-transfer behavior of vacuum residue during thermal conversion were studied. Firstly, the influence of colloidal stability of vacuum residue on the coke formation was studied through mass fraction normalized conductivity. Then the influence of hydrogen-transfer ability of vacuum residue on the coke formation during thermal conversion was tested in a batch type autoclave reactor by taking anthracene and 9,10-dihydroanthracene as chemical probes. At last, the effects of the addition of hydrogen-donor derived from FCC decanted oil and thermal conversion improvement additive (PCIA) on thermal conversion were tested.The results achieved are listed as follows:(1) The colloidal stability index and the relative colloidal stability index of vacuum residue were defined. And it was found that the colloidal stability of vacuum residue decreased remarkably with thermal reaction going on and the PCIA could effectively improve the colloidal stability of vacuum residue during thermal conversion. The colloidal stability of vacuum residue increases with increased content of PCIA.(2) Hydrogen-transfer abilities could be markedly influenced by reaction temperature and time. At the same reaction temperature, hydrogen-abstracting ability of heavy oil increased with reaction time, hydrogen-donating ability of the heavy oil increased and then decreased with time. At the same reaction time, hydrogen-transfer ability increased with temperature.(3) Hydrogen-abstracting reaction of a chemical probe in residue kinetically follows a first-order reaction model. The kinetics of hydrogen-transfer reaction was treated on two stages divided by taking the maximum of hydrogen-donating abilities.(4) The hydrogen-donating abilities of heavy oils and their sub-fractions decreased in the order of residue > resins > aromatics > asphaltenes > saturates, while the hydrogen-abstracting abilities increased in the order of asphaltenes > resins> residue> saturates≥aromatics during mild thermal treatment.(5) In views of hydrogen donating/abstracting transfer, the key factor controlling initial coke formation lied in hydrogen-donating abilities of resins and hydrogen-abstracting abilities of asphaltenes during residue thermal processing. Coke formation-index was defined by using hydrogen transfer abilities of heavy oils and could be used to explain the difference of coke-formation induction time of different heavy oils during thermal conversion.(6) The relative hydrogen-donating abilities of FCC decanted oil (F) and its sub-fractions (A,B,C,D) were different and decreased in the order of C>B>F>A>D. Because the active hydrogen could be liberated under the reaction conditions of thermal reaction, FCC decanted oil (F)and its sub-fractions (A,B,C,D) could be taken as hydrogen-donors during vacuum residue visbreaking, of which the sub-fraction C (fraction 460-500℃) of FCC decanted oil was more effective. The addition of these hydrogen donors into heavy oils increased, the yield of fraction below 180℃decreased, the yield of fractions at 180-350℃and the yield of fraction below 350℃increased, the viscosity of residual fuel oil at 100℃was reduced and the stability of residual fuel oil tested by spot test was improved. Because the relative hydrogen-donating ability of C was greater than that of F, the effect of C was better than that of F.(7) Because PCIA could effectively delay the coke formation e and activate the hydrogen-transfer during thermal reaction, it could enhance liquid yield and suppress coke yield during thermal reaction of heavy oils.(8) It was showed that the thermal visbreaking affected markedly the yield of products of heavy oils during two-stage thermal conversion (TSTC), i.e., a combined process of thermal visbreaking and delayed coking. There was an optimization between visbreaking and delayed coking. If the visbreaking processed reasonably, the liquid yield of two-stage thermal conversion would increase and the coke yield decrease in contrast with direct coking. The best visbreaking condition for Luoyang vacuum residue was 80 minutes at 400oC by using hydrogen-donor C at 15 percent and PCIA-0 at 150ug·g-1. When visbreaking processed at that condition, the two-stage thermal conversion could suppress coke yield by 2.0% and enhance liquid yield by 2.1% comparing with direct coking. (9) The ratio of (aromatics + resins)/(saturates + n-heptane asphaltene) was correlative to the colloidal stability of the residual oil processed by visbreaking. It was found that the better the colloidal stability of residual oil processed by visbreaking, the higher the total yield of liquid fractions and the lower the total yield of coke of TSTC.