Reaction Basis And Catalyst Design For Deep Hydrodenitrogenation Of Inferior Heavy Oil

In order to achieve the goal of green and low-carbon during the processing of heavy oil,and adapt to the changes in the market demand for vehicle fuel oil and chemical raw materials,and achieve the goal of high-quality development driven by innovation of refineries,refineries have to continuously improve the production capacity of heavy oil hydrotreating,and solve the technical problems encountered during the hydrotreating process,such as the slowly rate for the hydrodenitrogenation(HDN)of “low sulfur,high nitrogen” inferior heavy oil and the quickly catalyst deactivation.Therefore,based on the research on the reaction behavior of HDN,the investigation of the influencing factors of HDN,and the research on the inhibition effect of nitrogen compounds,the reaction basis of the deep HDN of inferior heavy oil was obtained.This could provide theoretical guidance for the design of hydrodenitrogenation catalysts.In this paper,the adsorption and conversion performances of nitrogen compounds and their hydrogenation intermediates were systematically studied by combining DFT calculations and hydrogenation experiments.The adsorption properties of nitrogen compounds with different structures on the active site are not only related to the charge density around the nitrogen atom,but also to the steric hindrance of the adsorption of nitrogen compounds on the active site.Compared with the non-basic nitrogen compounds with similar structures,due to the negative charge density around the nitrogen atom,the basic nitrogen compounds could adsorb on active sites through the nitrogen atoms only(end-on),and this would lead to a larger adsorption energy,which makes the the basic nitrogen compounds easy to adsorb and further hydrogenate.The alkalinity of the basic nitrogen compounds would weak after hydrogenation and saturation,and the adsorption steric hindrance of the hydrogenated six-membered nitrogen heterocycle on the active site is greater than that of the hydrogenated five-membered nitrogen heterocycle,which would weaken the adsorption and conversion performance of the hydrogenatedintermediates of the basic nitrogen compounds correspondingly weakened,and lead to the hydrogenation intermediates having the characteristics of “multiple types,high yields and difficult removal”.In addition,N-alkylation reaction of non-basic nitrides can occur during HDN process on alumina-based Ni Mo catalyst.For example,under experimental conditions,the yield of N alkylated derivatives in carbazole HDN products can reach up to 17.9%.The catalytic performance of nitrogen compounds is also related to the composition of nitrides in the raw materials.On the premise of a constant total N content,with the increasing of the ratio for basic nitrogen/non-basic nitrogen,the conversion rate of nitrogen compounds increases,but the denetrogenation rate decreases.The effects of H2 S,NH3 and nitrogen compounds on the hydrodesulfurization(HDS)activity and reaction path selectivity of dibenzothiophene(DBT)and 4,6-dimethyldibenzothophene(4,6-DMDBT)were studied.H2 S mainly inhibits the C-S bond breaking during HDS of DBT and 4,6-DMDBT,while NH3 mainly inhibited the hydrogenation reaction in the HDS of DBT and 4,6-DMDBT.When H2 S and NH3 coexist,the hydrogenation path(HYD)and hydrogenolysis path(DDS)of DBT and 4,6-DMDBT HDS were inhibited.Nitrogen compounds exhibited a stronger inhibitory effect on the HDS of DBT and 4,6-DMDBT.When the atomic ratio of nitrogen and sulfur is low(N/S < 0.30),nitrogen compounds mainly inhibited the HYD path;both HYD and DDS paths were strongly inhibited by nitrogen compounds at higher nitrogen/sulfur atomic ratio N/S > 0.30).To be specifical,basic nitrogen compounds(quinoline)and their hydrogenated intermediates exhibited different degrees of inhibition on the HDS of DBT and 4,6-DMDBT.1,2,3,4-tetrahydroquinoline(14THQ)and decahydroquinoline(DHQ)-type hydrogenation intermediates showed stronger inhibitory ability to HDS,while the nonbasic nitrogen(indole)hydrogenation intermediates in the HDN process had low yields,only the reactants themselves(indole)has a strong inhibitory effect on HDS,which led to that the inhibitory effect of basic nitrogen compounds on HDS is stronger than that of non-basic nitrogen compounds.The active phase of the HDN catalyst is the sulfided Ni Mo S phase,and the sulfur compounds would affect the state of the active phase and the active center.Studies showed that sulfur compounds have a promoting effect on HDN.More specifically,with the increase of sulfur content in the raw material,the conversion rate of nitrogencompounds increases,but denitrogenating rate first increases and then decreases,and reaches its maximum value at 1000 ppm of S.The promotion effect of sulfur compounds on HDN is also related to the structure of sulfide.The larger the molecular structure of sulfide,the smaller the promotion effect on HDN.The specific order is consistent with the HDS activity of sulfur compounds.The presence of sulfides also changed the path selectivity of HDN.Sulfur compounds promoted the ring-opening reaction of 14 THQ and DHQ(Csp3-N bond cleavage),indicating that the sulfhydryl group is the active site for Csp3-N bonds cleavage,while sulfur compounds inhibits the cleavage of Csp2-N bonds of propylanilines,indicating that the sulfur coordination unsaturated site is the active site for the cleavage of Csp2-N bonds.This phenomenon demonstrated that the active phase state on the catalyst surface is the key to the removal of nitrides(C-N bond breaking),and the active phase state is related to the concentration and structure of sulfide in the reaction system.In other words,the removal performance of nitrogen compounds in oil is closely related to the composition of raw materials.The basic idea of deep HDN of inferior heavy oil is that: priority hydrogenation saturation is the premise of HDN,and suitable isomerization and CN bond breaking ability are the guarantee of HDN.Therefore,the hydrogenation type and hydrogenolysis type HDN catalysts were prepared.The hydrogenation type catalyst was prepared by hydrothermal treatment of alumina precursor(pseudoboehmite),which effectively weakened the strong interaction between alumina and active metal,adjusted the Mo S2 stacking structure,and promoted the type II “Ni-Mo-S” active phase.When the hydrothermal treatment temperature of pseudoboehmite elevated to 180 oC,the propotion of Ni-Mo-S active phase over Ni Mo/A-180 can reach 83.2%.Compared with the reference(Ni Mo/Al2O3),the adsorption saturation and denitrogenation rate of quinoline on Ni Mo/A-180 increased by 23% and 78%,respectively.The lack Brónsted acid on the alumina surface results in insufficient hydrogenolysis capacity of Ni Mo catalysts supported on alumina.To solve this problem,10 wt.% Y molecular sieve was introduced into the alumina matrix material to prepare a hydrogenolysis-type HDN catalyst.At the same time,in order to improve the tolerance of Y zeolite to the poisoning effect of nitrogen compounds,three metals,including Ni,Fe and Zr,were used to modified Y zeolite by ion-exchange method.The introduction of Y molecular sieve effectively weakened the steric hindrance effect brought by the alkylsubstituent,among which Ni Mo/A-Zr Y exhibited suitable isomerization and good stability.In order to achieve a reasonable compatibility between the hydrogenation activity and the hydrogenolysis activity of the HDN catalyst,the hydrogenation type HDN catalyst and the hydrogenolysis type HDN catalyst were combined and packed.When the packing volume ratio of the hydrogenation type catalyst and the hydrogenolysis type catalyst was 2,under the reaction conditions of 380 oC,8 MPa,1.0 h-1,the denitrogenation rate and desulfurization rate in Liaohe coking wax oil(S: 10520 μg·g-1,N: 6060 μg·g-1)reached 86.6% and 95.6%,respectively.Compared with the conventional single catalyst(Ni Mo/Al2O3),its desulfurization rate and denitrification rate are increased by 10.3 and 22.1 percentage points,respectively,which has high industrial application potential.