Study On The Fluidized Bed For Ethylene Polymerization With Super Condensing Mode Operation

Gas-phase fluidized bed process is the major technology of polyethylene production due to the advantages as simple procedure, low investment, flexible operation, wide range of product category and so on, and playing an important role in polyethylene production field. The traditional non-condensation phase operation has a low space time yield (STY) of fluidized bed reactor because of the limitation of recycle gas stream’s capacity of reaction heats removal. Since the condensation phase operation is applied, there exists condensed liquid in recycle stream, so the polymerization heat is removed both by the temperature rise of recycle stream gas phase and the evaporation of the liquid phase, which leading to a increasing of the STY of fluidized bed reactor. However, the content of liquid phase in recycle stream is limited, so the reactor’s production capacity can’t be improved furthermore. To solve this problem and increase the liquid phase content, the super condensing process is necessary. At present the super condensing process is not mature yet, and the influences of liquid phase content increasing in recycle stream on fluidized bed operation, resin degassing unit and vent gas recovery unit are lack of deep research, so the super condensing technology is not applied widely. Thus, it is of great significance to study on the super condensing technology of gas-phase fluidized bed polyethylene process.This thesis has simulated the polymerization unit, resin degassing unit and vent gas recovery unit of gas-phase polyethylene process based on ASPEN software and mathematical modeling method, then proposed process improvement schemes using the simulation results. The purpose of the thesis is to develop a super condensing polyethylene technology. What’s more, the liquid atomization phenomenon is studied by acoustic emission (AE) technology, and a new detecting method of liquid atomization status is found. The main results in this thesis can be summarized as follow: Firstly, the polymerization unit model of gas-phase fluidized bed polyethylene super condensing process is established by using Polymer Plus software. The simulation results reveal the effect of reactor production capacity increase. Based on the model a modification scheme of fluidized bed recycle pipeline is proposed, that is, moving the compressor from cooler inlet to the gas stream pipeline after the gas-liquid separator, and the new process can reduce the load and power of the compressor.Secondly, the resin degassing unit mathematical model of gas-phase fluidized bed polyethylene process is established based on diffusion law, henry law, conservation equation and so on. The relationships among various variables are investigated by using the degassing unit model, including:with the increase of nitrogen flow rate, the outlet concentration of volatile components increases quickly at first, then tends to be stable; prolong the residence time is beneficial to resin degassing process; residence time and nitrogen flow rate have opposite effects to resin degassing process when the outlet concentration of volatile components is limited; decrease the pressure of purge bin can enhance the degassing performance; the concentration of volatile components decrease gradually from the top of purge bin to the bottom; the concentration gradient in the upper is greater than that in the lower. Then the operation curve of purge bin and the optimized operation condition can be obtained.Thirdly, the vent gas recovery unit of gas-phase polyethylene process is simulated using Aspen Plus software, and three modification schemes of vent gas recovery unit are proposed. From the simulation result of traditional compression condensation recovery unit, it is found that increase compressor outlet pressure and decrease condensing temperature can promote the recovery of vent gas. Membrane gas separation models are established in Aspen Plus based on the transfer models of gas separation membrane. A new vent gas recovery unit based on gas separation membrane module is put forward, and the simulation results show that the new unit can improve the recoveries of all hydrocarbon components. Then a new vent gas recovery unit based on condensed liquid circulation loop is proposed, and the simulation results show that the new unit can improve the recoveries of low boiling point components like ethylene. At last, a new vent gas recovery unit based on gas separation membrane module and condensed liquid circulation loop is proposed. In the new recovery unit, the recoveries of various hydrocarbon components can reach90%or above, and the recovery of ethylene increase greatly. In addition, the tail gas can be returned to purge bin as purge gas, so the flow rate of flash nitrogen can be reduced. Through comparison the new recovery unit based on gas separation membrane module and condensed liquid circulation loop is the optimized modification scheme.Fourthly, the liquid atomization status is studied by acoustic emission method. The generating mechanism and character of the AE signals, and the characteristic scale (J1) of AE signals which represented the status of liquid atomization were obtained by using power spectrum analysis, wavelet transform and R/S analysis. The liquid atomization status can be judged by the variation of energy fraction of AE signals on J1scale.