Study On Preparation Of Needle Coke Carbon Material From Heavy Coker Oils

Needle coke carbon material is widely applied in both steel making and aerospace industries. However there is a severe supply shortage of this material in China, which is mainly due to an insufficient source of its immediate production material. Therefore centering on the two key concerns encountered in the production of needle coke material, i.e. the mechanisms of needle coke production material pretreatment and the ensuing carbonization / cocarbonization, this investigation delved into the mechanisms of thermal cracking pretreatment and the ensuing carbonization and cocarbonization of heavy coker oils. First, thermal cracking pretreatment mechanism of a heavy vacuum coker gas oil (HGO) was investigated in some detail, followed by carbonization of thus derived product gas oil. Then, carbonization behavior of another heavy coker oil, FCC decant oil (DO), was also studied. Finally based on the findings from the investigations afore, laboratory and pilot tests on the cocarbonization of the both feedstocks were carried out in turn to prepare needle coke. Cocarbonization mechanism was revealed and the performance of the needle coke obtained was analyzed.HGO was rich in aromatic ring structures with low contents of sulfur and ash. However, it was high in alkyl carbon fraction. These characteristics made it promising to obtain a premium feedstock for needle coke production after thermal cracking pretreatment. When HGO underwent thermal cracking, the product gas oil concentrated aromatic hydrocarbons compared to the parent HGO. Thus from the viewpoint of being used as a needle coke feedstock, its molecular compositions and structures were greatly improved. At the same time, alkyl carbon chains present in HGO were converted into high value-added products such as gasoline and diesel fuel. By using FT-IR technique during HGO thermal cracking, the methylene to methyl numerical ratio (NCH2/NCH3) of saturates underwent an increase first and then a decrease. For the aromatic fractions (i.e., aromatics and resins), they got a gradual decrease in NCH2/NCH3, and a gradual increase in both aromatic hydrogen fraction and hydrogen to carbon atomic ratio of the aromatic ring sheet.Carbonization and cocarbonization of DO and the optimized vacuum gas oil derived from HGO thermal cracking were common in that they had their individual optimum processing conditions. With the reactions going on, there witnessed a gradual decrease in maltene yield, an increase first and then a decrease in both asphaltene and toluene insoluble– pyridine soluble (PS) yields and thus forming a maximum in the middle, and an ongoing increase in coke (pyridine insoluble) yield. Coke formation was the result of the sequential conversion of malteneâ†'asphalteneâ†'PSâ†'coke. At the same time in the reaction process, product gas evolution rate was crucial to forming needle coke of well developed flow-domain textures. Coke formation could be depicted by overall first order kinetics. However, there existed sharp distinctions among these carbonization/cocarbonization processes. As far as the kinetic parameters of coke formation of these processes were concerned, coke formation from DO carbonization was the slowest, while that from cocarbonization was moderate at a high temperature and the fastest at lower temperatures (460~480oC). This showed that at lower temperatures, there existed synergism during cocarbonization of the two feedstocks, leading to accelerated coke formation.The vacuum gas oil derived from HGO thermal cracking pretreatment could be used as premium feedstock for needle coke production through cocarbonization. If it was cocarbonized with premium needle coke feedstocks, then this category of feedstock source could be effectively expanded. If it was cocarbonized with common needle coke feedstocks, then the quality of needle coke as a product could be improved with its feedstock sources expanded. Therefore by thermally cracking HGO and taking the derived product vacuum gas oil as needle coke feedstocks, the aromatic ring sheet present in HGO molecule could be converted into high value-added needle coke with the production of liquid distillate as a high value-added by-product. In this way a novel upgrading processing technology for efficient utilization of HGO has been fashioned, through which catalyst premature deactivation or poisoning due to overdosing HGO into traditional upgrading processes such as FCC or hydrocracking units could be avoided, thus leading to promising benefits for both the economy and the society.