The Study On The Endothermic/Exothermic Effect During Thermal Process Of Heavy Oils

Heavy oils with different origins would lead to considerable differences in thecomposition and chemico-physical properties, and then different performances in the thermalprocess, especially the endothermic/exothermic effect. Therefore, it is necessarily importantto study the endothermic/exothermic effect in depth in order to optimize thermal balanceprocess and improve the energy efficiency. However, the basic data regarding the thermaleffects of heavy oils was scarce, and corresponding research has not been systematicallyimplemented yet. The goal of the thesis is thus to study the thermal effects and the correlationbetween that and structure properties of heavy oils based on thermal performance of severaltypical heavy oils (i.e. petroleum residues) in order to lay the foundation of optimization ofthermal process of heavy oil.In this thesis, Differential scanning calorimetry (DSC) combined with thermalgravimetric analysis (TGA) has been used to creat a thermal analysis method for quantitativediscriminating thermal effects in thermal process. The thermodynamics and thermokineticshave been then studied based on the method. Results showed that the coke yield has a crucialimpact on the enthalpy of reaction during thermal process of petroleum residues. Thatindicates that it would require more time and less energy than the others to complete thepyrolysis process of inferior residues. The endothermicity in pyrolysis mainly refects a heatrequirement for pyrolysis instead of evaporation. The order of activation energy of thethermal processes for four petroleum vacuum residues is: Daqing vacuum residue(DQVR)>Karamay vacuum residue (KRVR)>Liaohe vacuum residue (LHVR)>Venezuelanvacuum residue (VNVR). The order also indicates that pyrolysis enthalpy is easiest to vary inthe case of VNVR. It is implied that the1.5order reaction model is appropriate for thedescription of whole pyrolysis process of heavy oils. The enthalpy of thermal process of residue fractions was also studied. Results indicatedthat the whole reaction ranges of saturates, aromatics, and resins are shown as beingendothermic, which means pyrolysis dominate the thermal process. However, the intensityand distribution of endothermic heat vary as the feed changes, while the temperature at whichendothermicity reaches the maximum level is the same (around500℃). The asphaltenesfrom LHVR and VNVR both show exothermic behaviors throughout the process on thecontrary. The exothermic curves occur in three partially overlapping steps, showing threepeaks in the range from425to475℃. The values of fA+fNhas important influence on thereaction enthalpy of thermal process of petroleum residues. It could indicate that thethermodynamic characteristic of petroleum-derived feedstock would change fromendothermic to exothermic when the coke yield rises to some point, which is49.1wt%inthis study.Furthermore, microscopy, micro calorimetry with DSC model, and UV-vis were used forthe study of the interaction between fractions of residues. Results demonstrated that Thetemperature range for the effective peptization of dispersion medium is from roomtemperature to300℃. Dispersion medium fractions of residue with different structureproperties have different peptization effects on the asphaltenes. The peptization effect couldbe adjusted to improve the stability of colloidal system by adding proper compoundsaccordingly. Peptization effect would not be the crucial factor that determines theperformance in thermal process of residues when temperature is over300℃. Properselection of blending petroleum feedstocks would reduce the coke yield, promote the cokequality in addition to controlling the formation of shot coke through hydrogen transfer effect.Furthermore, the inhibited coking resulted from hydrogen transfer effect has more obviousinfuences on the endothermic enthalpy in the thermal process.At last, the effect of blended oils on the thermal processing of residues has beenresearched. The main results showed that the blending system of DQVR and VNVR has arelatively bad compatibility, which could cause phase separation easily. That resulted incoking in different phases in the coking process of blending feedstock. The blending proportion of inferior VR had better be under20%. Compared to LHVR, VNVR would makethe variation of thermal heat intensity and the shift of thermal heat in the thermal processmore severe. For the sake of smooth operation of thermal process and economicalconsideration, heavy gas oil with recycled ratio of0.1-0.3used as recycled oil in thermalprocess would be valid and acceptable.