The Studies On The Diffusion Coefficient And Henry's Constant Of Volatile Components In Viscous System

In polymer process,the volatiles' residual content in the final product is a significant indicator to evaluate the product quality.In the industrial process,the devolatilizer is usually used to remove the volatiles.when the pressure in the devolatilizer is higher than the foam devolatilization pressure of the material,the devolatilization process is mainly dominated by diffusion devolatilization,and the devolatilization rate is influenced by the kinetic factor(diffusion coefficient)and thermodynamic factor(Henry's constant)collectively,which thereby are important parameters of diffusion devolatilization.In this paper,the volatiles' diffusion coefficient and Henry's constant in n-alkane/polydimethylsiloxane(PDMS)solution system under different conditions are determined by experimental method,and the influence of operating conditions on both coefficients are discussed.The volatiles'diffusion coefficient is determined by combining their desorption curves in the liquid membrane by gravimetric analysis with the mathematical model based on the Fick's second law.The results show that the diffusion coefficient of the n-heptane decreases with the increase of system viscosity or the decrease of its mass fraction,when the viscosity of PDMS increases by 14.7%,the diffusion coefficient decreases by 16.0%and when the mass fraction increases from 5%to 20%,the diffusion coefficient increases by 19.4%;in addition,the diffusion coefficient of the n-alkane decreases with the increase of its molecular weight and the diffusion coefficient of the n-pentane is 1.9 times as large as the n-octane's.The Vrentas-Duda model can describe the relationship between the diffusion coefficient and the volatiles' mass fraction and molecular weight well.The Henry's constant can be determined by the relationship between the mass fraction of the solution and its corresponding equilibrium partial pressure.The results show that when the temperature increases from 303.15 K to 343.15 K,the Henry's constant of the n-heptane increases to 4.8 times;the diffusion coefficient of the n-alkane decreases with the increase of its molecular weight and the Henry's constant of the n-pentane is 16.7 times as large as the n-octane's;the influence of system viscosity on Henry's constant is non-significant,and the Henry's constant of the n-pentane only increases by 13.7%when the PDMS viscosity increases to 95.8 times.Finally,the diffusion behavior of incondensable/condensable gas in PDMS was investigated by molecular simulation.It can be found that the diffusion coefficient of the n-alkane in PDMS shows good agreement with the value of experiment or literature by pcff force field and Ewald non-bond calculation method,the structure of PDMS has obvious influence on the value of the diffusion coefficient and the computational stability and accuracy are improved in larger simulation system.