Basic Study On Engineering Of Heavy Oil Catalytic Pyrolysis For Maximizing Propylene Production

On the basis of two-stage riser fluid catalytic cracking technology advantages, two-stageriser heavy oil catalytic pyrolysis for maximizing propylene production（TMP） achievedmore propylene and high quality of gasoline and diesel. But some questions need to be furtherimproved and optimized in its process of industrialization. In this paper the influence ofreaction conditions, conversion distribution of the two stages, matching of the feed,optimization of subsequent absorption-stabilization system were researched in detail, toprovide theoretical guidance for commercial application of the technology.A series of experiments were carried out in the small FCC experimental unit. The resultsindicated that high temperature, high ratio of catalyst to oil and long residence time werebeneficial for increasing heavy oil conversion and propylene yield. However, dry gas yieldincreases and light oil yield reduces with the temperature elevation and extension of residencetime. Because of the characteristics of TMP technology, high heavy oil conversion andpropylene yield can still achieved even if under the condition of relatively low temperatureand short residence time. It can also suppress the dry gas yield to ensure the light oil yieldwell. Recycling gasoline can not only increase propylene yield, but also reduce olefinsconcentration in gasoline. The gasoline quality is improved in this process.By using Daqing AR as feed and LTB-2catalyst, the experimental results showed thatpropylene yield achieved22%, and butylene yield achieved20%. The yield of diesel oil andgasoline were both about18%. Total yield of gasoline, diesel and LPG was82.36%. Thequality of diesel oil produced by catalytic pyrolysis of heavy oil in two-stage riser is alsoacceptable. The density of mixture of diesel oil from the first stage riser and the second stageriser was about890kg/m3. Calculated cetane number was about30.Based on the product distribution of catalytic pyrolysis of Daqing AR in the small FCCexperimental unit, an8-lump kinetic model was developed. Parameters of this model wereestimated by using sub-model method, and apparent activation energies were calculatedaccording to the Arrhenius equation. The predicted yields coincided well with the experimental values. The model was used to calculate the TMP with two stage riser andrecycling gasoline. TMP can obtain high propylene yield, while maintenance diesel oil yieldat the relax reaction conditions. Recycling C4can increase propylene yield greatly. If thegasoline was recycled in the second stage riser, the conversion of heavy oil in the first stageriser should be controlled at about70%, and the total conversion should be controlled at about90%.According to heat equilibrium, the relationship between operation temperature andcomposition of the feedstock of C4fractions and fresh oil, coarse gasoline and recycle oil inTMP technology were calculated through a series of association models based on thesimulated distillation data of the liquid products and analytical data of gas products. Thereaction heat was calculated based on the combustion heat of different fractions, and theproduct distribution was calculated based on the established catalytic cracking eight lumpingdynamic models and that programmed calculation was realized. The calculation showed thatin the first riser, as the riser outlet temperature increased, the catalyst-oil ratio and the recycleratio of C4fractions increased gradually, and when the riser outlet temperature maintainconstant, the catalyst temperature at the entrance of raw materials increased, the catalyst-oilratio decreased, and the recycle ratio of C4fractions reduced correspondingly. In the secondriser, the variation of catalyst-oil ratio and the recycle ration of gasoline had the similar trendas the first riser, but under the same conditions, the catalyst-oil ratio of the second riser waslarger.Based on the empirical method of separation sequence determination the absorption andstability-gas separation combined unit process was determined. On this basis, a new stabilitysystem of low pressure absorption process was proposed, and the optimal operatingconditions of the process were determined by process simulation software. Compared with theconventional processes, the amount of cooling water and steams can be saved, in whichsaving cooling water equivalent to2.31kg standard oil/t raw material (40.81%), saving1.0MPa low pressure steam8.86kg of standard oil/t raw material (38.52%), saving3.5MPastandard high-pressure steam177.76kg oil/t raw material (24.32%).