Numerical Simulation On Hydrogenation Of Acetylene In Slurry Bed Reactors

The process of using coal as raw material to make calcium carbide,then producing acetylene by calcium carbide,and finally processing ethylene through acetylene hydrogenation can expand the source of ethylene and is an important prospective ethylene production process.Slurry bed reactor has great potential in the process of acetylene hydrogenation to ethylene.At present,the knowledge of acetylene hydrogenation in slurry bed reactor is still limited.Therefore,in-depth study of acetylene hydrogenation process in slurry bed reactor is of great significance to the scale-up,design and industrial application of the reactor.In this study,the multiphase flow,mass transfer and reaction characteristics of acetylene hydrogenation in the slurry bed reactor were analyzed by numerical simulation.Firstly,considering that the existence of catalyst particles in the slurry bed makes the bubble size larger,leading to the reduction of drag force between the gas and liquid,a drag force model suitable for describing the gas-liquid interaction in the slurry bed was first established.Considering the three-phase flow characteristics of gas,liquid and solid in the slurry bed,a multiphase flow model in the slurry bed was established based on the Eulerian-Eulerian multifluid scheme.The three-phase flow characteristics of gas,liquid and solid in the slurry bed were simulated using the multiphase flow model.The results show that the presence of catalyst particles in the slurry bed increases the size of turbulent eddy in the liquid phase,restrains the breaking of bubbles,and enhances the coalescence of bubbles,so that the size of bubbles in the bed becomes larger,resulting in the decrease of gas holdup in the bed and the distribution uniformity of the multiphase flow in the radial direction.When the superficial gas velocity increases,the backmixing of gas,liquid and solid in the slurry bed increases,and the distribution uniformity of the multiphase flow characteristics decreases in the radial direction,and the effect of particles is more significant at higher gas velocity.Then,according to the characteristics of multiphase flow and reaction in the slurry bed reactor for acetylene hydrogenation,the multiphase flow model and acetylene hydrogenation reaction kinetics were coupled to develop a comprehensive flow-reaction mathematical model.The model was used to simulate a pilot-scale slurry bed reactor,the liquid phase is N-Methyl pyrrolidone(NMP)and the catalyzer is Pd/Al₂O₃.The results show that the radial distribution uniformity of gas holdup increases with the presence of the vertical tube internals.The concentration of acetylene decreased along the axial direction,the conversion of acetylene increased along the axial direction,and the conversion of acetylene has already reached a high level at the lower part of the reactor(near the inlet).In radial direction,acetylene concentration is larger in the center of the reactor,but smaller near the wall.The ethylene concentration increased along the axial direction,and the ethylene selectivity increased first and then decreased along the axial direction.The distribution of ethylene concentration in radial direction is opposite to that of acetylene.The influence of operating conditions(slurry height,superficial gas velocity,and ratio of hydrogen to acetylene)on the process of acetylene hydrogenation in the slurry bed reactor was investigated by using the comprehensive flow-reaction mathematical model.The results show that increasing the slurry height or reducing the superficial gas velocity can provide more time for the acetylene hydrogenation process,so as to effectively improve the conversion of acetylene,but also make the time of ethylene hydrogenation longer,so as to reduce the selectivity of ethylene.In addition,increasing the ratio of hydrogen to acetylene in feed gas can also improve the conversion of acetylene,but the selectivity of ethylene will be significantly reduced.For the reactor of this paper,considering both acetylene conversion and ethylene selectivity,the ratio of hydrogen to acetylene in feed gas should be controlled in the range of 4.45:1?6.00:1,and by adjusting the slurry height or superficial gas velocity in the reactor,the reaction time should be controlled in the range of 18?23 s,which can achieve higher acetylene conversion rate(?96%)and ethylene selectivity(?90%).Finally,the comprehensive flow-reaction mathematical model was used to further investigate the influence of the vertical tubes(external diameter and arrangement of vertical tubes)on the flow,mass transfer and acetylene hydrogenation reaction in the reactor.The results show that at constant heat-exchange surface area,with the external diameter of the vertical tube increasing in a certain range,the gas holdup in the reactor increases,the backmixing of the gas and liquid at the wall weakens,and the multiphase flow uniformity in the reactor increases.With the increase of the external diameter of the vertical tube,the effect of the tube on the turbulence of the liquid phase is weakened,the turbulent kinetic energy of the liquid phase in the reactor is increased,and the contact area between the gas and liquid is increased,which enhances the mass transfer process between the gas and liquid in the reactor.Therefore,increasing the external diameter of vertical tube in a certain range can enhance the hydrogenation process of acetylene.The conversion of acetylene in the reactor is significantly increased,and the selectivity of ethylene is only slightly reduced.For the reactor of this paper,it is more appropriate to choose the external diameter of the vertical tube at 32 mm or above,which can achieve higher acetylene conversion(?96%)and ethylene selectivity(>91%).The effect of vertical tube arrangement on the overall flow,mass transfer and reaction characteristics is smaller than that of the external diameter of vertical tube,and only has limited impacts on the local flow,mass transfer and reaction characteristics.The results show that when the arrangement of vertical tubes in the central region of the reactor is compact and sparse near the wall,it can restrain the bubble from gathering to the central region of the bed,and make the multiphase flow characteristics more evenly distributed in the radial direction,thus facilitating the reaction.