Study On Phase Equilibrium And Flow Phase Transition Of Impurities In Heat Exchanger Of Air Separation Unit

Raw material of the air separation unit contains a lot of impurities,some of which cannot be effectively adsorbed,and precipitate to form crystals during liquid oxygen evaporation process in the main condenser-evaporator and liquid oxygen subcooling process in the subcooler.Among them,the easiest to crystallize include nitrous oxide,carbon dioxide and acetylene.The precipitated crystals gradually accumulate on the wall of the heat exchange channel,and then forms a flow dead zone which may cause the pipeline to block.The collision of some crystalline particles with the wall causes the local temperature rise in the channel to exceed the ignition point of hydrocarbons,which may result in accidents such as explosions.Based on the phase equilibrium model,this paper studies the crystallization mechanism and physical properties of impurities in the heat exchange channel,and analyzes the influence of crystallization on flow and heat transfer characteristics and the influence of impact between crystal and wall,so as to provide guidance for the optimization of air separation process.The results of this paper are as follows:1)Based on the fugacity coefficient and activity coefficient,the PR-MHV-NRTL model is established,which is verified by experimental data of gas-liquid phase equiblirium of propane-sulfur dioxide mixture under 283-343 K and nitous-sulfur dioxide mixture under 278-333 K.The prediction error of nitrogen-sulfur dioxide density is less than 7.5%.Through this model,the existing gas-liquid and liquid-solid experimental data of nitrous oxide-oxygen and carbon dioxide-oxygen are correlated,and the correlation error is less than4%.The physical properties of the mixture are calculated and the mixture crystallization discriminant formula is established.2)A numerical model of nirous oxide-oxygen phase transition in the serrated plate-fin heat exchange channel is established.The precipitation and growth laws of crystallization under different working conditions are studied,and the discriminant formula of crystallization critical condition is established.The precipitation and growth laws of crystallization are studied,and the discriminant of crystallization critical conditions is established.When the heat flux density,the outlet temperature or the inlet mass flow density is greater than the critical vaule,there is a risk of crystallization.Crystals mainly begin to precipitate from the front and rear ends of the fins and the back ends of the staggered joints near the wall,and gradually grow inward after stable accumulation.3)The heat transfer coefficient of the fluid is mainly affected by the thermal resistance of crystalline solid and heat flux density,and the system pressure drop is mainly affected by the effective flow area of the flow channel.When the crystal does not precipitate,it has few effect on the flow characteristics.After the accumulation of crystal,the thermal resistance is greatly increased,and the effective area of the channel is reduced,therefore the system pressure drop increases.When the inlet temperature and mass flow density remain unchanged,the channel heat transfer coefficient increases first and then decreases as the heat flow density increases,and the system pressure drop presents an overall upward trend.When the inlet temperature and heat flow density remain unchanged,the fluid heat transfer coefficient and system pressure drop show an upward trend as the mass flow density increases.4)Based on the explicit dynamic model,a temperature rise model for particle collision with the wall is established,and the vertical collision process with the largest temperature rise is studied.The model was verified by the existing metal particle experimental data,and the model was transferred to the crystal,and the process of the crystal particle and the wall of the heat exchange channel was studied.The temperature rise has a cubic relationship with the particle diameter and a quadratic relationship with the initial velocity of the particle.Under the working conditions of the air separation unit,nitrous oxide particles with a diameter of 4822 μm collide vertically with the wall at an initial velocity of 10m/s,which can ignite acetylene particles,resulting in accidents.In the case of high concentrations of nitrous oxide and ozone,the required size is smaller.