Promoting The Hydroconversion Of Multicyclic Aromatics To Monoaromatics:Mechanism Study And Process Development

The key problems for upgrading Chinese diesel are high ratio of inferior light cycle oil(LCO),high aromatics content,low cetane number of diesel pool and its great density,which leads to greater difficulty for upgrade when comparing with foreign diesel.Upgrading LCO needs hydrosaturation of a large amount of aromatics,high hydrogen consumption and harsh operation conditions,which will surely cause great increase of investment and energy consumption.Therefore,inferior LCO processing is the bottleneck for China petrochemical industry to upgrade diesel with high efficiency and low consumption.For diesel distillate,high aromatics content is unfavorable.However,naphtha distillate with high aromatics content can be used as gasoline blending component with high octane number.So UOP and SINOPEC both have developed corresponding hydroconversion technologies for LCO and have achieved partial conversion of polycyclic aromatic hydrocarbons(PAHs)in LCO.This can not only utilize the aromatics rich in LCO to produce gasoline with high octane number,but can also improve diesel quality and reduce the difficulty of diesel upgrade for refineries.In this article,the study object is the production process of high octane number gasoline from hydroconversion of LCO;the emphasis is the solution of problems,such as poor selectivity of target product in industrial application of FD2G technology,high hydrogen consumption,mismatch of the temperatures between the outlet of hydrorefining reactor and the inlet of hydrocracking reator,long adjusting period after the startup,etc.Main conclusions were listed as follows:1.Based on experiments,selective hydrocracking network was proposed for monoaromatics formation with napthalene(model compound for dicyclic aromatics)and phenanthrene(model compound for tricyclic aromatics)as starting material.Thermodynamic equilibrium calculation and impact analysis of reaction conditions are conducted for this complex reaction network.From thermodynamic point of view,it was proposed that operation with low pressure and high temperature could increase the selectivity of hydroconversion from PAHs to monoaromatics.The results showed that in order to increase the production of monoaromatics,the preferred reaction temperature was between 400 ? to 420 ?,with as low as possible the reaction pressure in the range of testing,while the amount of participated hydrogen was related to the feedstock.The optimum molar ratio of hydrogen to naphthalene was 4,while the optimum molar ratio of hydrogen to phenanthrene was 8.2.For industrial NiMo/HY catalyst,kinetic models for naphthalene and phenanthrene selective hydrocracking were established respectively,and the parameter values of the model were estimated employing nonlinear least square method in combination with experimental data of reaction kinetics.Through statistical analysis(t-test and F-test)of model parameters and comparison with experimental data,the reliability of the reaction kinetics model was verified.The comparison among the constants of reaction rate in the network showed that end-aromatic ring was much easier to undergo hydrocracking reaction compared with aromatic ring in the middle of PAHs.From kinetic point of view,it was proposed that high reaction temperature favored the selective hydroconversion of PAHs to monoaromatics.The established selective hydrocracking kinetic model of napthalene and phenanthrene was of important reference value for development and upgrade of LCO hydroconversion technology.3.The correlation between hydrocracking catalyst type and hydroconversion of LCO was put forward.In order to improve the selectivity to monoaromatics,the isomerization and ring-opening ability of catalyst should be intensified,thus hydrocracking catalyst with weak hydrogenation activity and strong acidity was suitable for this process.Through the impact study of process conditions on hydroconversion of LCO,the optimum operation range was settled,which was approximately 8MPa of pressure and high temperature(usually beyond 390'C).The obtained method can be used to improve the existing technologies,which features high monoaromatics selectivity and low hydrogen consumption in LCO hydroconversion process.4.Based on the impact study of molecular sieve content on catalyst properties for LCO hydroconversion,it was found that the acid strength is not in direct proportion with acid content.Appropriate amount of molecular sieve should be employed to increase the acid strength of the catalyst.More acidic sites is not necessarily better,since it will cause coking thus having effect on catalyst stability.In the scope of the experimental conditions,the catalyst with 50%molecular sieve content was with the best properties.At comparable conversion,the aromatics content of the obtained distillate with boiling point less than 200 ?from hydroconversion has increased by 1.4%;Its octane number has increased by 1.8 units,and the chemical hydrogen consumption has reduced by 0.12%.The purpose to improve the target product hydroconversion selectivity from LCO and to reduce the hydrogen consumption was reached.From long cycle experiment,the stability of the catalyst was verified.5.Based on the above-mentioned study results,the scenario analysis for industrial application of the hydroconversion technology of LCO was conducted.Operation method with variant temperatures and pressures was developed,which solved the problem of long adjusting period after the startup.The volume space velocity of hydrorefining catalyst was proposed to be raised to solve the problem of temperature mismatch between the outlet of hydrorefining reactor and the inlet of hydrocracking reactor at the preliminary stage of startup.The performance study of FC-24B catalyst in LCO conversion proved that the deposit carbon deactivation on the catalyst surface is responsible for the phenomenon that both the octane number of the gasoline product and liquid yield of the unit are low at the beginning and gradually increased as the running time prolonged.6.The possibility of processing LCO with ebullated-bed process was explored,and its comparison with fixed bed was conducted.The experimental results showed that the ebullated-bed process could further improve the selectivity of the target product,reduce hydrogen consumption and produce gasoline product with better octane number.Meanwhile,due to the unique backmixing performance and on-line function of catalyst replacement,long running cycle could be achieved.In addition,for ebullated-bed reactor there was no need for cold hydrogen injection between the beds as in the case of the fixed bed,which lead to better utilization of reaction heat.