Fundamental Studies On Catalytic Cracking Of Coker Gas Oil

Fluid catalytic cracking (FCC) is one of the most important processes convertingheavyoil into light oils. With the rapid increase of coker gas oil (CGO) and the qualityof FCC feed changing worse and worse, it has become an important way for mostrefineries to increase profit that processing more CGO in FCCU. However, due tocontaininghigh content ofnitrogencompounds, poly-aromatics and resins, theblendingratio of CGO in FCC feed has been limited. Therefore, study on catalytic cracking ofCGO and seeking the technical measures for solving the problem are of greatsignificance.Inthepaper,the catalytic crackingofnitrogencontaining compoundsandits effecton ZSM-5 and USY catalysts were studied. The results showed that operationparameters, such asreaction temperature, catalyst/oil ratio and residence time all couldsignificantly affect the nitrogen distribution in products. Hereby, the higher reactiontemperature and catalyst/oil ratio, and shorter residence time could be beneficial inreducing the nitrogen poisoning. And the results also revealed that, although the USYcatalysts showed higher conversion of CGO than the ZSM-5 catalysts, the ZSM-5zeolite could obstruct the lager-molecule basic nitrogen compounds of CGO fromentering the catalyst pores, thus reducing its susceptibility to poisoning. Furthermore,the catalytic cracking of CGO over ZSM-5 and USY catalysts was comparativelyinvestigated. The results showed that the poor crackability of CGO and the deleteriouseffect of nitrogen compounds on the performance of cracking catalysts were the mainreasons which lead to hard cracking of CGO. Though the catalystswith higher activitycouldbefavorable for enhancingthe conversionofCGO,theyalso couldresult inmorecoke. Nitrogen compounds in CGO preferentially and easily were adsorbed and/orcondensed to coke during the initial contact with catalyst, thereby harmful tohydrocarbons cracking. Meanwhile, the hydrocarbon composition or hydrogen contentofCGOwasthemainfactorwhichcouldgreatlyaffectedtheyieldofpropylene.The catalysts with low selectivity of coke for catalytic cracking of CGO, that is USY/SiO2/La2O3 and USY/Al2O3/Kaolin, was successfully developed, through themodificationofacidityandimprovingtheporestructureofmatrix,respectively.A large amount of experiments had been carried out using CGO and commercialFCC catalyst in laboratory FCC unit. Compared with the conventional catalyticcracking process, TSRFCC could enhance the conversion of CGO by more than 9percent points. However, to obtainthe near conversion, the conventional FCC had to beoperated under the very rigorous reaction conditions, which could deteriorate theproduct distribution. For the TSRFCC technology with the diesel separation from oilvapor of the first stage riser, the yields of diesel and light oil could be significantlyincreased,and the ratioofdiesel to gasolinecould also beenhanced. Ifthe gasoline wasrecycled to the second-stage, then the olefin content can be reduced by 15-22 points,while the anti-knock index of product gasoline changed little. Futhermore, usingTSRFCC technology could be realized that the catalytic cracking of fresh FCC feed-stock and CGO took place in separate riser, which could prevent the detrimentalcompetition of adsorption and reaction existing between them, and thus made its bigadvantagesandhighflexibilityinprocessingCGO.Maximizing yield of propylene by two-stage riser catalytic cracking of FCCfeedstock blending CGO was proved to be feasible. The experimental results showedthatusing the LTB-2 special catalyst bleding 30% FCC catalyst, the yield of propylenereached 20%, the yield of LPG was above 40%, and the total liquid yield andconversion was also high, while the yield of dry gas and coke was below 12%. If thebutene was recycled, the yield of propylene would be more higher. In addition, thegasoline from the second stage riser contained less olefins and more aromatics, andwhichwasa highoctanenumbercomponent.