Study On Catalytic Cracking Of Feedstock With High Content Of Aromatic Compounds And Nitrogen-containing Compounds

Fluid catalytic cracking(FCC)plays an important role in petroleum refining process for converting heavy oil into light products.With the decreasing of conventional feedstocks,it becomes a promising way to process heavier and more inferior feedstocks in order to get great economic benefits.However,due to high content of aromatic compounds and nitrogen-containing compounds in heavy and inferior feedstocks,the yield of desired products and the activity of catalyst are seriously affected,which is a great challenge for FCC.In this paper,Liaohe coker gas oil(CGO)with high content of aromatic compounds and nitrogen-containing compounds was used as raw material.Based on accurate characterization of nitrogen-containing compounds and aromatic compounds which were key retardation components,the transformation of nitrogen-containing compounds and the retardation effect of aromatic compounds were studied aimed to provide guidance for FCC to improve the processing of feedstocks with high content of aromatics and nitrogen-containing compounds.In this work,Liaohe CGO was distilled into six narrow fractions by true boiling point distillation.The basic properties and catalytic cracking performance of these narrow fractions were investigated.The results showed that as a product from secondary process,although the main characteristics for different CGO were similar,some specific properties might be different.As fractions became heavier,their catalytic cracking performance tended to be poorer.One reason was that the degree of catalyst deactivation was different.Another reason was that the amount of components with good cracking property reduced with the increasing boiling point.Nitrogen-containing compounds in narrow fractions and their catalytic cracking liquid products were characterized by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry(ESI FT-ICR MS).The results showed that N1 class species(a compound contains one nitrogen atom,similarly hereinafter)had the highest relative abundance in all narrow fractions,some nitrogen class species existed discontinuously.Compared to N1 basic nitrogen-containing compounds in the same fraction,N1 nonbasic nitrogen-containing compounds had narrower mass distribution,shorter side chains and smaller molecular weight.After catalytic cracking reaction,the molecular weight of nitrogen-containing compounds became smaller and their distribution became more concentrated.N1 class species tended to have higher condensation degrees and shorter side chains.Compared to basic nitrogen-containing compounds,nonbasic ones had weaker adsorption capacity,but it was still easy for them to condense into coke on the catalyst.In composite heteroatom compounds,heteroatom-containing structures were more prone to crack into small molecules.On the condition that tetralin was hydrogen donor,the catalytic cracking performance of quinolone was investigated.Based on the characterization of soluble coke,it was proved that quinoline was able to hydrogenated into 5,6,7,8-tetrahydroquinoline,then cracked into pyridine during the catalytic cracking reaction.Aromatic compounds were separated from narrow fractions and characterized by comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry(GC×GC-TOF MS).Finally,their retardation effect on Daqing vacuum gas oil(VGO)was investigated.The results showed that their adverse effect was mainly from two reasons.One was that aromatic compounds were hard to convert into smaller ones.Another was that their strong ability of coke formation.With the increasing aromatic rings of blended aromatic compounds in Daqing VGO,the intensities of dehydrogenation condensation,monomolecular cracking and thermal cracking reactions increased.The facilitating degree of dehydrogenation condensation from aromatic compounds with no less than three rings was similar.Aromatic compounds with no more than four rings had similar promoting effect on monomolecular cracking reaction.In the whole reaction,catalytic cracking kept dominant.