Steady-State Analysis And Dynamic Simulation Of Gas Fractionation Unit

The gas fractionation process is an important way to obtain polymer-grade propylene in the refinery process.In the refinery production process,the catalytic cracking and coking unit operation changes cause the liquefied gas production and propylene composition to fluctuate frequently,causing fluctuations in the gas fractionation unit,resulting in unstable control of critical variables such as temperature and pressure of the device.This not only has a serious impact on the yield of propylene products,but also has an important effect on the control of propylene purity.In addition,the propylene tower in the unit is prone to driving instability during the start-up process and to causing flooding accidents,which seriously threatens the safety of the production process.Therefore,it is significant to the analyze the dynamic characteristics and response of the unit under disturbance and accident,investige the reliability of control strategy,improve the emergency treatment level in sudden accidents,obttain safe and reliable process parameters,protect the product quality and improve the efficiency for company through dynamic simulation.In this paper,the steady-state simulation and hydraulic calculation of a gas fractionation unit in a petrochemical company were carried out by Pro/II software,and the basic process parameters required for dynamic simulation were obtained.For the serious problem that the deviation of the relative volatility prediction of the near-boiling system predicts a large deviation from the theoretical plate number prediction,the experimental data is used to correct the interaction parameters of propylene-propane and the results showed that the simulation results of the propylene tower are more consistent with the real production data after the revision,which lays a foundation of the reliability of dynamic simulation.The DYNSIM software was used to construct the dynamic model of the deethanizer tower,and its dynamic response under various disturbance conditions was analyzed.The discharge of the safety valve in the top of the tower was analyzed under fire,condenser cold source cutoff and electricity power failure accident.The conclusion is that the discharge time under the fire accident is the longest,but the discharge amount is stable,and the discharge time of the condenser cold source cutoff accident is the shortest,but the instantaneous discharge amount is large,and the temperature of the tower kettle rises fastest when the electricity power failure occurs.The dynamic simulation of the propylene tower was carried out,and its dynamic characteristics were investigated.The advantages and disadvantages of the two control modes(cascade control and thermal bypass control)of the pressure in the top of tower were compared.The results showed that the reliability and stability of the thermal bypass control mode are better;the influence of the pressure drop of the liquid phase relay pipeline in the upper and lower towers of the propylene tower on the vaporization rate was analyzed.In order to make the unit start stably and operate normally at this composition and temperature,the critical pressure drop of the pipeline should not exceed 33 kpa,and the flow rate should not exceed 1.9m/s.The effect of heating capacity of propylene tower on the liquid level of the reflux tank and tower kettle was investigated.It is found that when the heating amount is less than 497.373 t/h or greater than 680.615 t/h,the normal and stable operation of the tower will be seriously affected.