Structure and Composition Changes of Nitrogen Compounds during the Catalytic Cracking Process and Their Deactivating Effect on Catalysts

The comprehensive structure and composition changes of the nitrogen compounds during the catalytic cracking processes of coker gas oil and vacuum residue are investigated using electrospray ionization combined with Fourier transform ion cyclotron resonance mass spectrometry. These experiments were conducted over different cracking materials under the reaction temperatures of 500/520 degrees C, the weight hourly space velocity of 18 h(-1), and the catalyst/oil ratio of 5. The results show that the diffusion resistance in the micropores of the zeolite is the key factor affecting the interaction between the nitrogen compounds and the acid sites. The basic N1 and N2 class species with double bond equivalence (DBE) values smaller than 10 can easily diffuse into the micropores of the zeolite and are preferentially adsorbed onto the acid sites. These adsorbed nitrogen compounds generally conduct condensation reactions and hydrogen transfer reactions to form coke deposited on the cracking catalysts. The basic N1 and N2 class species with DBE values larger than 10, other basic nitrogen compounds other than N1 and N2, and the non-basic nitrogen compounds seldom interact with the acid sites of the zeolite. They usually undergo side chain thermal cracking on the surface of the matrix, which can reduce their carbon numbers but cannot change their DBE values. The basic N1 class species with DBE values smaller than 10 are the main compounds that poison the cracking catalysts. In comparison to the SL-CGO catalytic cracking, the nitrogen-poisoning effect on the catalysts is much less during the SL-VR catalytic cracking process because the main poisoning compounds (the basic N1 class species with DBE values smaller than 10) are much fewer.