Difficult Separations

Difficult separations using distillation techniques depend primarily upon the nature and properties of the components in the mixture, rather than the physical arrangement of the equipment such as bubble caps versus sieve trays, tray versus packed columns, etc. Reactive distillation is an exception to this generalization (see next section). Distillation contrasts with other separation techniques such as membranes and adsorbents where the characteristics of the actual separation media have a significant effect upon the ability to separate the components. This is why generic distillation methods have been developed and used effectively whereas there is no single theory or method available for the design of either membrane or adsorption systems. However, distillation was developed in order to separate mixtures of components exhibiting significant differences in relative volatilities and boiling points and problems arise where this is not the case. In particular, azeotropes exhibit no difference in volatility (at certain conditions) and hence no change in the composition of the mixture obtained. Traditional approaches have been either to avoid the conditions where an azeotrope forms, or (more likely) the addition of a solvent or entrainer which 'breaks' the azeotrope but which also requires an additional column(s) to remove and recycle the solvent. The common azeotropes are well known and documented, e.g. ethanol/water, acetone/chloroform, etc. Recent developments have centred on the ability to predict the occurrence of an azeotropic mixture (or a mixture with very little difference in component volatilities), and also on the application of the traditional design methods to the design of a range of column configurations. These configurations aim to produce an optimum separation in terms of product specification, minimum use and recycle of solvent, effective and feasible control schemes, minimum capital and operating costs. Computer-based property packages have been developed specifically to predict azeotrope formation, and the vapour-liquid equilibria data can then be used in an appropriate column simulation package to evaluate the alternative equipment arrangements. Researchers have developed prediction and property packages by the application of basic thermodynamic principles, and also utilizing specific equations of state (see section on VLE data above). The property packages have also included data on available solvents. The aim is generally to combine azeotrope prediction with solvent selection, and full specification of the combined mixture properties for use in column design simulations.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment