Structure Design And Scale-up Test Of Hydrocracking Y Zeolites With High Selectivity To Middle Distillate

In recent years, the demand for middle distillate has been increasing, and the requirements for its quality have also been improving. But environmental regulations are more and more rigorous, clean technologies are urgently critical to produce clean fuels. Hydrocracking is feasible to produce middle distillate directly from VGO, which shows many advantages, such as even flexible in processing, better quality of product and high middle distillate yield. Catalyst is the key to hydrocracking process. The acidity and pore structure of catalyst determines the yield of middle distillate. USY zeolites have large specific surface and adjustable acid property and thus are potentially applied in increasing middle distillate yield as hydrocracking additives. But the industrial USY could not be used in industry cause the mesoporosity is too poor and the acid strength and density is too high, which usually leads to seriously secondary cracking. Herein, industrial USY zeolite was tentatively modified by citric acid and ammonium dihydrogen phosphate(MAP)or ammonium fluoride(NH4F)to achieve couple or combined dealuminization. This method focused on the changes of acidity and mesoporous structure and its potential to creat USY rich in mesopore which could decrease the diffusion resistance of macromolecular and improve the efficiency of internal pore system, so as to meet the requirements of hydrocracking catalysts with high selectivity to middle distillate.Coupled or combined modification of USY using organic acids and inorganic salts is seldom reported. In this study, two systems, citric acid-MAP and citric aid-NH4F, were investigated. After investigating the coupled or combined effect and individual effect of modification systems, the optimum charging order were obtained and the initial modification mechanism was postulated. The results indicated that the modification was influenced by the concentration of citric acid and MAP or NH4F, liquid-liquid ratio, reaction temperature and reaction time. L27(313) orthogonal layout with five factors and three levels was designed and their orthogonal interactions were also included. The optimum modification condition was obtained from variance analysis and direct comparison analysis for citric acid-MAP and citric aid-NH4F, respectively. According to the optimum condition, scale-up test was done in 1 L autoclave. The structure and properties of modified USY were characterized by means of BET, FT-IR, NH3-TPD, XRD and TG-DTA. It was found that the specific area, volume of secondary pore, and the amount of medium strong acid as well as Si-Al ratio were all improved, and at the same time, the modified USY could maintain good crystallinity and thermal stability. The modified samples of scale-up test were used as cracking component to prepare hydrocracking catalysts, Performance evaluation was carried out on a 200 mL fixed-bed single stage hydrogenation unit using Daqing VGO as feedstock. The middle distillate selectivity of two USY samples could reach up to more than 80%. Compared with industrial catalyst, the yield and selectivity were both increased. In addition, these methods had wide applicability for other industrial USY raw materials and could achieve good modification results. It showed that coupled or combined modification could meet the need of hydrocracking to produce middle distillate in the industrial scale and it might be a potentially industrial process to get mesoporous Y zeolite.