Mixing And Mass Transfer Characteristics And Reactor Model Of Packed Bubble Column In Liquid Phase Hydrogenation Process Of Gasoil

The non-circulating gas-liquid upflow liquid phase hydrogenation technology of gasoil is a new chemical engineering process to produce low-sulfur gasoil.This process can not only reduce the sulfur content of gasoil effectively,but also save a large amount of equipment and operation cost,showing a huge potential for the petrochemical refining industry.The essential point of this process is to feed in pre-saturated gasoil,and the hydrogen needed for reaction comes from the dissolved hydrogen in the liquid oil.The hydrogen consumed in the reaction is then supplemented by a continuous multi-point injection.Immediate and adequate hydrogen supply is key to this process.Based on non-circulating gas-liquid upflow liquid phase hydrogenation technology of gasoil,two aspects were addressed in this dissertation.On one hand,by measuring hydrogen solubility in straight run gasoil under the industrial conditions,technical parameters in this process were determined.On the other hand,the mass transfer and mixing conditions in the packed bed bubble column were studied by cold model experiment.Finally,these two parts and establish a reactor model were combined together,guiding the design of hydrogen injection and scale up of packed bubble column.The main contents and achievements of this thesis are as follows:(1)The solubility of hydrogen in straight run gasoil under industrial conditions was measured by sampling flash evaporation method.The variation of hydrogen solubility with operating temperature and pressure was investigated.Through the experimental data,an empirical correlation was proposed to calculate the hydrogen solubility in straight run gasoil,with relative deviation to the experimental values less than 5%.At the same time,the gas solubility simulation process was established by Aspen plus software,and the effects of impurity gas as H2S,NH3 and CH4 on hydrogen dissolution were investigated.It was found that the hydrogen solubility increases with the increase of the operating temperature and pressure,while the existence of impurity gas tend to inhibit the hydrogen dissolution,and the strongest inhibition effect shown in the experiment is CH4,NH3 followed,and H2S the weakest.(2)The variation of gas-liquid mass transfer coefficient with gas velocity,liquid velocity,and packing particle diameter in the packed bubble column was investigated by means of dissolved oxygen electrode method.The results showed that under the experimental conditions,increasing the gas and liquid velocity can both increase the gas-liquid mass transfer coefficient,while the effect of liquid velocity is more significant.The gas-liquid mass transfer coefficient decreases first and then increases with the increase of the packing particle diameter.Through the analysis of the experimental datum,an empirical correlation formula for gas-liquid volume mass transfer coefficient kLa calculation was obtained.The relative deviation between the predicted values and the experimental values is within±20%.(3)The variation of the residence time distribution of the liquid phase with gas velocity,liquid velocity and packing particle diameter was investigated by using the electrolyte tracer method.The results showed that the average residence time decreases with the increase of liquid velocity,but not so sensitive to the change of gas velocity.When the packing particle diameter is small,the average residence time decreases with the increase of the packing particle diameter.However,when the packing particle diameter exceeds a certain value,increasing the packing diameter has no significant influence on the average residence time.Based on the residence time distributions under different operating conditions,the axial diffusion model was established to describe the backmixing degree in the reactor.The results showed that the backmixing degree decreases with increasing liquid velocity and increases with increasing gas velocity.In the meantime,the backmixing degree decreases first and then increases with the particle diameter.Through the regression analysis of the experimental data,the empirical correlation between the Peclet number,gas and liquid Reynolds numberand packing particle diameter has been proposed.The relative deviation between the predicted values and the experimental values is within the range of ±20%.(4)Based on the results of experimentally measured hydrogen solubility,and the empirical correlations for the gas-liquid mass transfer coefficient and the backmixing degree,the mathematical model of the packed bubble column was established according to the operating characteristics of the non-circulating liquid phase hydrogenation process of gasoil.The calculated results agreed well with the reference values.The effect of the input flow temperature,space velocity as well as the hydrogen partial pressure on the hydrodesulphurization efficiency and the temperature difference of the reactor was investigated.The results showed that the hydrodesulphurization efficiency increases with increasing temperature of input flow and decreasing space velocity.When the volumetric hydrogen-oil ratio is over 100 Nm3·m-3,input temperature over 360?,and space velocity LHSV less than 2.0 h-1,the hydrodesulphurization efficiency will not change with the input temperature as well as space velocity.Increasing temperature of input flow or decreasing space velocity can both increase the temperature difference of the reactor.Increasing hydrogen partial pressure has no significant effect on the hydrodesulphurization efficiency,but increased the temperature difference of the reactor a lot.According to the principle that the temperature difference of the catalyst bed should be less than 12? and between the inlet and outlet less than 20?,the design scheme of the number and location of hydrogen injection point in the liquid phase hydrogenation process of gasoil was investigated.It showed that at constant volumetric hydrogen-oil ratio,setting one or two hydrogen injections can both decrease not only the temperature difference but also the hydrodesulphurization efficiency.In order to ensure high hydrodesulphurization efficiency,the hydrogen-oil ratio in the injection part should be less than 20 Nm3·m-3,and it should be located near the bottom of the catalyst bed.