Study On The Apparent Column Efficiency Of Industrial Propylene-Propane Distillation Process

Distillation is recognized as the most mature homogeneous separation technology at present,which has a relatively mature theoretical basis and operational experience after nearly a hundred years of development,but it is still at the level of semi-empirical and semi-scientific disciplines.The biggest barrier to the development of distillation technology is the accurate prediction of tray efficiency.The study of the improvement of the tray efficiency can not only promote the development of subject,but also help to realize the energy saving in the process of industrial distillation.Tray efficiency is a very complex function of system physical properties and operating equipment,and its definition is not unique.The biggest significance of accurate prediction of tray efficiency for industrial application is to reliably evaluate the apparent column efficiency,so that it can reach the efficiency range of engineering design reference and improve the success rate of design.Propylenesplitter is the main unit to produce high purity propylene in chemical industry.Because of the close boiling points between propylene and propane,the actual number of C3 splitter is generally more than 180 actual plates.In this paper,owing to the current difficult problem of reliable evaluation of Ea for C3 splitters(often more than 100%from the evaluation results),the prediction errors of Ea were divided into the technological variable(the number of theoretical stages)and the equipment variable(overall column efficiency Eoc and the number of actual stages),and the influence of these two types of prediction errors were quantified,then the reliable evaluation of Ea for C3 splitters were finally realized.The essence of NT is the process of measuring how many equilibrium stages are required for measuring the separation requirements on the scale of phase equilibrium.Therefore,the prediction accuracy of phase equilibrium property directly determines the accuracy and credibility of NT calculation.In this paper,the phase equilibrium of propylene-propane system was studied,and a new phase equilibrium correlation method(COSMO-RS+PR)was developed.Compared with the database(multi-group phase equilibrium data),the average prediction deviation of system pressure was no more than0.18%,and of propylene gas phase molar composition was no more than 0.48%.The prediction accuracy of this method is the best in a wide temperature range(230～350 K),and the strange variation trend of relative volatility of this system in the high concentration region of propylene has been accurately described.Based on this method,the non-ideal characteristics of the system in the high concentration region of propylene were further studied.By considering the non-ideality of liquid phase and gas phase separately,it was found that when T≈310 K,the non-ideality of the system was the lowest and closest to the ideal state in the high concentration region of propylene.Based on the experimental data of density,the variation trend of excess molar volume(V m~E)was obtained.It was found that the intermolecular attraction and repulsive force of the system could be approximately offset when T≈310 K in the high concentration region of propylene,and then Vwas equal to 0.It was proved that the lowest nonideality of the system at T≈310 K was accurate,and the trend that the relative volatility increased at first and then decreased with the increase of temperature in the high concentration region of propylene was explained successfully for the first time.On the basis of the phase equilibrium study of propylene-propane system,the influence of relative volatility prediction deviation on NT calculation error of C3 splitters under total reflux and partial reflux were quantified.According to the quantitative results,applying the stage-to-stage calculation method,the rigorous and accurate model for calculating NT of C3 splitters was established.This model had three main characteristics:the assumption of the relative volatility was variable across the column,the assumption of the molar overflow was variable;the assumption of ratio of vaporization heat between two components was variable across the column.Through these three assumptions,the model was more rigorous and more accurate than other NT calculation models,which provided the standard value for NT calculation of C3 splitters.Based on this standard value,the errors of calculating NT for C3 splitters applying other shortcut methods were compared with each other,and a new shortcut method was given.Applying the shortcut method in this paper,the average prediction deviation was around1.5 theoretical stages,which provided support for accurate and rapid estimation of NT calculation of C3 splitters.On the basis of the accurate NT predictions of the proposed method,the prediction errors of NT for C3 splitters design were quantitatively analyzed and discussed using Aspen Plus.It provided guidance for designers to select appropriate methods corresponding to different actual operating conditions for the design of C3splitters,and also provided a theoretical basis for the adjustment of operating conditions of the actual column.On the basis of accurate calculation of NT for C3 splitters,the applicability of various predictive Eoc models for reliable evaluation of Ea was screened.Compared with the production plant data,the calculation error of the actual stage number was about three plates.It was proved that the difficulty of reliable evaluation of Ea for C3 splitters was that the accuracy of Eoc models was accurate enough.On the other hand,there was a large deviation in the prediction of NT for C3 splitters.Summing up the safety factors of reduction design,the scheme and guideline of reliable evaluation of C3 splitters were finally given.Through the process of reliable evaluation of Ea for column equipment in this paper,it provided a breakthrough point to solve the problem of accurate prediction of tray efficiency.