Synthesis And Simulation Of Reactive Divided-wall Column For Methylal Production

Methylal is an important raw material that can be used for production of many chemical intermediates,solvents,paints and fuel additives.The high purity methylal can be synthesized by the extractive reactive distillation process(ERDP)with water as extractant,which can be classified into two types according to the source of extractant:the double-feed process that utilizes water in formaldehyde solution directly and the three-feed process that introduces another water feed stream.Although both processes have been certificated and investigated by experiments,the deeply analysis and comparison between them still has been nonexistent.In addition,the excess reactant,i.e.methanol,should be recycled in the process,hence it's meaningful to find a way to reduce the process cost.In this work,both two kinds of extractive reactive distillation process for the methylal production were simulated and compared in order to find out which process is better.Then,the reactive divided-wall column(RDWC)for producing methylal was synthesized and simulated,and the characteristics and transfer resistances of the RDWC process were also analyzed and evaluated,respectively.The main contents of this work were listed as follows:(1)The double-feed and three-feed extractive reactive distillation process for producing methylal were simulated and optimized based on the appropriate thermodynamic and reaction kinetic model,and the difference between the two processes in reaction,separation and economic were deeply analyzed and compared.The results showed that both process can produce high purity methylal as expected.Although the three-feed process can promote the separation between methanol and methylal,the reaction rate and reboiler duty were reduced and increased respectively due to introduction of extra water.The comparison on total energy consumption and TAC between the two processes indicated that the three-feed process was better than double-feed process.(2)Based on the above work,the reactive divided-wall column(RDWC)for producing methylal was synthesized and analyzed.The results indicated that the RDWC can avoid the backmixing of methanol that occurred in the conventional process,and this was the main reason that the RDWC can lead to energy reduction.The simulated results showed that the three-feed RDWC process can reduce the energy consumption and TAC by 22.2%and 19.01%respectively in comparison with the double-feed conventional reactive distillation process,and it's also the most economical one in all processes studied in this work.(3)The transfer resistances related to the liquid-vapor interphase and catalyst of the three-feed RDWC process for producing methylal were evaluated and analyzed,respectively.The results showed that the transfer resistances between liquid-vapor interphase had significantly impacts on the composition,temperature and gas-liquid flowrate distribution of the column,and the pressure of both sides in the RDWC can also be influenced,which is an important influence factor on the performance of the RDWC.Moreover,the heat transfer of the divided wall in the RDWC can also be affected by the transfer resistance between the liquid-vapor interphase.Lastly,the transfer resistance related to catalyst was evaluated.The calculated average catalytic efficiency in the reactive section was 0.86,and it's found that the composition and temperature distribution in the reactive section changed a little after considering the catalytic efficiency.Also,this effect was essential for the design of RDWC for methylal production.