Fundamental Study On Thermal Conversion Of Inferior Heavy Oil With Hydrogen Donor

With the reserves of conventional crude oil and the production scale depleting worldwide, the exploitation and upgrading of unconventional oil resources has gained great attention. Inferior ultra-viscous and ultra-heavy oil, as unconventional resource, is characterized by high viscosity, high density, high heteroatom content, high asphaltene content and metal content, which brought great challenges to its exploitation, transportation and processing. Venezuelan crude oil is a typical inferior heavy oil. How to effectively upgrade this kind of viscous oil is the technical difficulty which the production department and transportation section are confronted with and is also the hotspot in research and development area.In this paper, fundamental study on visbreaking(VB) of Venezualan heavy oil and thermal conversion of Venezuelan heavy oil with hydrogen donor(HVB) was carried out, which laid the basis for the development of hydrogen-donated thermal cracking technology. Based on the systematic study on visbreaking of Venezuelan vacuum residue(VVR), improvement on an evaluation method for hydrogen transfering ability(Chemical probe method) has been made. The modified method was used to screen two hydrogen donors DA and DB and systematically evaluate their hydrogen transfering ability. Hydrogen donor visbreaking of VVR in the presence of hydrogen donors DA and DB was then investigated and the kinetic model was built. The analysis of the hydrocarbon compositions of DB before and after reaction and the effect of DB on the average structural changes of residual asphaltenes revealled the essence of hydrogen transfer reaction and hydrogen donor’s mechanism of action.By visbreaking of VVR, the viscosity of the upgraded oil could be effectively reduced and it satisfied the specifications of pipeline and tank transportation. However, its stability turned out to be poorer, which couldn’t satisfy the requirements(50°C kinematic viscosity lower than 380 mm2·s-1 and spot test rank better than Rank 2) of pipleline transportation, tank transportation and storage.The research by modified chemical probe method showed that, the hydrogen trasnferring ability of DB was stronger than DA. The colloidal stability of VVR with DB during the thermal reaction was better than VVR with DA.Based on visbreaking of VVR, the results from thermal conversion of VVR in the presence of hydrogen donors DA and DB showed that the sort order of upgrading effects was VVR+DB>VVR+DA>VVR, and the optimum technology condition for thermal conversion of VVR in the presence of hydrogen donor DB was 425°C, 5min, at which it could make its upgraded oil satisfy the needs of tank transportation. At the same coke yield 0.1%(mass ratio), the sort order of viscosity reduction rate and stability was VVR+DB>VVR+DA>VVR. The microscopic oberservations of the residue reaction system showed that the VB and the HVB reaction systems both had obvious phase changes. The hydrogen donor inhibited the growth of amorphous micro-particles to postpone the phase separation, made the coke induction period delayed and stability of visbroken oil improved.In comparison with the distillate yield in the conventional VB process, that in the HVB process was raised by about 4.4 percentage point(mass ratio, the same hereinafter) at the same spot rank. The kinetic modelling results showed that compared to convention VB, the reaction rate of VVR to gas, gasoline, diesel and light vaccum gas oil was lower and the corresponding activation energy was higher, which revealed why the hydrogen donor mitigated the thermal conversion of VVR and incresed its reaction severity.According to the changes of hydroaromatic content in the hydrogen donor DB before and after reaction, the calculation model of apparent transferred hydrogen in the HVB process was established, which revealled the essence of hydrogen transfer reaction.With the reaction time increasing at 425°C, changes of residual asphaltenes from visbroken residuals(VBR) and hydrogen donor visbroken residuals(HVBR) were as follows. The average molecular weight of VBR and HVBR asphatlenes increased first and then decreased. The alpha and beta hydrogen(Hα+Hβ) content and gama hydrogen Hγ content were gradually reduced while the aromatic hydrogen HA content was raised. The aromatic carbon rate f A of the asphaltenes increased and their total ring number RT and aromatic ring number RA both increased first and then decreased. In comparison with VBR asphaltenes obtained at the same reaction time, average molecular weight of HVBR asphaltenes were lower, however, the H/C atom ratio and(Hα+Hβ) content were higher. The hydrogen donor played the role of reducing the aromatic carbon rate f A of asphaltenes and increasing the naphthenic carbon rate f N and the ratio of naphthenic ring number to aromatic ring number RN/RA so as to delay the phase separation and coke formation.The HVB of VVR in the autoclave tests provided the operation parameters for lab continuous apparatus tests and these operation parameters were verified by the continuous tests, which laid the foundation for the operation of industrial plant. The industrial plant tests indicated that the upgraded oil from the HVB process satisfied the requirements for pipeline and tank transportation. Its viscosity reduction rate was over 98% and it could be stably stored over 6 months.