| Industrial gases are widely used in various manufacturing and processing industries,of which oxygen and nitrogen are the most used air products.At present,the preparation of oxygen nitrogen gas is mostly carried out by cryogenics distillation method,which has the advantages of high product purity and mature technology.However,due to the close boiling points of oxygen and nitrogen,there are problems such as high separation energy consumption and high equipment investment costs,which restrict the further reduction of production cost.At present,through the development and application of structured packing,the efficiency of cryogenic distillation has been greatly improved,but the pressure drop and mass transfer loss in the rectification process are still very large,and its energy consumption can account for 30%-50% of the total energy consumption of the system.The relative volatility of oxygen and nitrogen is relatively low,so better mass transfer performance is required than distillation in petrochemical and other industries.The current distillation efficiency optimization methods mainly includes improving equipment structure,optimizing process design and coupling with other energy-saving technologies.Improving the mass transfer efficiency through the traditional method of increasing specific surface area will greatly increase the pressure drop,and to enhance the efficiency of the system by enhancing the gas-liquid heat and mass transfer process with a limited tower diameter and pressure drop is an urgent task.Therefore,it is necessary to further solve the key scientific issues such as bubbling in the cryogenic distillation process,falling film flow and mass transfer mechanism,as well as the new rectification enhancement mechanism.Based on the new method of magnetic-field-assisted cryogenic distillation proposed in this topic,this paper explores the potential breakthrough point of enhancing the gas-liquid momentum and heat and mass transfer process through magnetic field,and reveals the influence mechanism of magnetic field on gas-liquid two-phase from theoretical simulation and experimental research.Three aspects of work has been carried out as the following:(1)The movement and mass transfer process of the bubble starting from stillness under the buoyancy in the non-uniform magnetic field constructed by the electromagnetic field is studied,through the finite element numerical calculation,and the control effect of the magnetic field on the gas-liquid interface motion is revealed.When the bubble passes through the magnetic field area constructed by the electromagnetic field,as the direction of the magnetic field force is opposite to the direction of gravity,the rising speed of the gas in the magnetic field is suppressed.When the magnetic field increases,it will stop rising and even move in the reverse direction.In a magnetic field with a central magnetic induction intensity of 0.193 T,the average velocity of the 2.6 mm bubble moving to the center of the magnetic field is slowed by 14%.When the direction of the magnetic field force is the same as that of gravity,the bubble accelerates and the liquid phase disturbance is significantly enhanced,which increases the bubble oscillation.The velocity vortex formed by liquid oxygen in the flow field can accelerate the diffusion of the components in the liquid phase,demonstrating the enhancing effect of the magnetic field on mass transfer process.(2)Using the finite element numerical calculation method,the multi-physics coupling simulation of the liquid-oxygen two-phase bubbling behavior under the magnetic field is studied.The breaking effect of liquid-phase circulation on the gas-liquid interface is revealed.When the magnetic field of the permanent magnet is added in the flow field,the high gradient magnetic field at the edge of the permanent magnet triggers liquid phase circulations.A low pressure area appears in the center of the circulation,and the gas flow path is changed.The direction of the circulation also has a significant effect on the velocity of the gas.At the bottom of the flow channel,the liquid phase moves from both sides to the center,and then merges at the gas phase inlet to promote the gas velocity to further increase.The enhancement of the liquid phase turbulence makes the gas-liquid interface easier to break.In the magnetic field with a maximum field strength of 0.47 T,the inlet velocity is 0.1 m / s,when the gas phase is bubbled for 0.3 s,the gas-liquid interface in the flow field reaches twice as much as that without magnetic field.Changing the size of the circulation can increase its fluidity,and can further enhance the disturbance of the downstream liquid phase.At the same time,it is expected to destroy the heat and mass transfer boundary layer near the wall and further improve the heat and mass transfer efficiency.(3)Through the cryogenic flow visualization experimental platform,the rising velocity and diameter distribution of the bubble group in liquid nitrogen during the ascent process are obtained.Oxygen-nitrogen gas-liquid mass transfer experiments under different inlet flow rates with and without magnetic field are carried out,which reveal the bubble size distribution under magnetic field,reflecting the influence mechanism of magnetic field force on the gas-liquid interface and transfer process.Through image analysis,the nitrogen bubbles in liquid nitrogen are mostly distributed between 0.74-2.67 mm,and their distribution meets approximately normal distribution,and60% of the bubble diameters are distributed between 1.22-1.93 mm.The experiment found that when the bubble in liquid nitrogen started to rise from rest,and its velocity was mainly distributed between 0.1-0.3 m/s.Also,drag coefficient increased with the increase of Reynolds number.In the further experiments of oxygen-nitrogen liquid mass transfer at different inlet flow rates with or without magnetic field,the particle size of the bubbles becomes smaller and the number increases when the magnetic field is added.When the inlet oxygen flow rate is less than 4 g / min,the bubble diameter distribution peak drops from [3.48,4.75] mm to [0.90,2.18]mm;when the inlet oxygen flow rate is greater than 4 g / min,the distribution peak of bubble diameter drops from [1.25,2.13] mm to [0.37,1.25] mm.There is an optimal inlet flow rate of5 g / min that makes the gas-liquid mass transfer time the shortest and the heat and mass transfer efficiency the highest.After the magnetic field is applied,the mass transfer time is reduced by 25%.The enhancing mechanism of the gas-liquid mass transfer and the liquid-gas contact area breaking phenomenon by the magnetic field in the experiment will launch a theoretical foundation for the further research and design of the magnetic field assisted distillation technology. |
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