乙腈-水共沸体系的变压精馏模拟与优化

利用Aspen Plus化工模拟流程软件对乙腈-水共沸体系进行变压精馏模拟分离研究。选择UNIQUAC物性计算模型确定变压精馏的工艺流程,通过灵敏度分析模块分别考察高压塔和常压塔的进料板位置和回流比对分离效果的影响。模拟结果表明,当塔操作压力为350kPa,塔板为30块,进料板为第10块塔板,回流比为1.5,在塔底可以得到质量分数为99.7%的产品乙腈。     

Dissolution and passivation of iron in acetonitrile and acetonitrile–water mixtures

The dissolution and passivation of iron in neutral acetonitrile–water mixtures have been studied under potentiodynamic and potentiostatic conditions. In “dry” acetonitrile (water content <0.02% or 200 ppm), passivation, due to an air-formed film, is maintained for a wide range of potentials up to 0.50 V, well above the corrosion potential E_corr≈−0.4 V. Transpassive dissolution at higher potentials is normally controlled by interfacial reactions rather than diffusion in the solution. Addition of a relatively small amount of water (0.5% or 0.28 M) to acetonitrile initiates active dissolution on iron surfaces damaged by previous transpassive dissolution. At a level of 2% water (1.1 M) active dissolution is initiated on undamaged surfaces and proceeds relatively slowly under control by interfacial reactions. Transpassive dissolution at similar potentials meanwhile becomes diffusion-controlled. As water content is increased further (e.g., to 6% or 3.3 M) the electrochemistry of iron becomes progressively similar to that in aqueous solutions, with both active dissolution and passivation being enhanced. Passivation is promoted by addition of hydrogen peroxide as a passivator, but the dynamic balance between the two processes can, as expected, be shifted to dissolution by increasing solution acidity. The similarities and differences in acetonitrile, water and their mixtures are discussed in terms of the relative reactivity and concentration of the two solvents. The profound effects of water on the kinetics of iron dissolution and passivation are attributed to the dominant reactivity associated with acidic hydrogen. Based on the potentiostatic and potentiodynamic features, the mechanism of phase change during dissolution is described in terms of a model of a shifting oxide film formed through two-dimensional nucleation and growth.