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1
Basic Concepts in Distillation
1.1 Introduction
Distillation is a thermal separation method for separating mixtures of two or more substances into its component fractions of desired purity, based on differences in volatilities of componentsāwhich are in fact related to the boiling points of these componentsāby the application and removal of heat. Note that the term distillation refers to a physical separation process or a unit operation. Remarkably, distillation can be combined with another distillation operation, leading to a dividing-wall column (Harmsen, 2010), or with a chemical reaction, leading to reactive distillation (Sundmacher and Kienle, 2003; Sundmacher, Kienle, and Seidel-Morgenstern, 2005; Luyben and Yu, 2008; Sharma and Singh, 2010), and/or other chemical process operations (Schmidt-Traub and Gorak, 2006).
At the commercial scale, distillation has many applications, such as the separation of crude oil into fractions (e.g., gasoline, diesel, kerosene, etc.), water purification and desalination, the splitting of air into its components (e.g., oxygen, nitrogen, and argon), and the distillation of fermented solutions or the production of distilled beverages with high alcohol content (Forbes, 1970). Distillation underwent enormous development due to the petrochemical industry, and as such it is one of the most important technologies in the global energy supply system (Harmsen, 2010). Essentially, all transportation fuel goes through at least one distillation column on its way from crude oil to readily usable fuel, with tens of thousands of distillation columns in operation worldwide. In view of the foreseen depletion of fossil fuels and the switch to renewable sources of energy such as biomass, the most likely transportation fuel will be ethanol, methanol, or derivatives. The synthesis of alternative fuels leads typically to aqueous mixtures that require distillation to separate ethanol or methanol from water. Consequently, distillation remains the separation method of choice in the chemical process industry. The importance of distillation is unquestionable in providing most of the products required by our modern society (e.g., transportation fuel, heat, food, shelter, clothing, etc.).
The analysis, design, operation, control, and optimization of distillation columns were studied extensively in the last century but, until the introduction of computers, only hand calculations and graphical methods were developed and applied in distillation studies. As distillation analysis involves many iterative vaporāliquid phase equilibrium calculations, and tray-to-tray component balances that are ideal for digital computation, the use of computers has had a beneficial effect in recent decades (Luyben, 2011). Many companies still have their own in-house process simulators, although commercial steady-state and dynamic process simulators (e.g., Aspen PlusĀ®, Aspen DynamicsĀ®, ChemCAD, Aspen HYSYSĀ®, PRO/II, etc.) are now available and dominate the fieldāwith distillation playing a key role in these simulators.
The topic of distillation is very broad and it would require many volumes to cover it in a comprehensive manner. Consequently, for more details the reader is directed to several good books, which cover this subject in great detail: Kister (1992a), Kister (1992b), Taylor and Krishna (1993), Stichlmair and Fair (1998), Seader and Henley (1998), Doherty and Malone (2001), Mujtaba (2004), Petlyuk (2004), Lei, Chen, and Ding (2005), and more recently Luyben (2006, 2011).
It is important to note that distillation can separate chemical components only if the compositions of the vapor and liquid phases that are in equilibrium with each other are different. Therefore, a practical understanding of vaporāliquid equilibrium (VLE) is essential for the analysis, design, and control of distillation columns. This introductory chapter presents in a structured and convenient way the basic concepts of distillation: property methods, vapor pressure, bubble point, relative volatility, VLE, vaporāliquidāliquid equilibrium (VLLE), ternary diagrams, residue curve maps (RCM), and theoretical stage and short-cut design methods for distillation.
1.2 Physical Property Methods
An extremely important issue in distillation calculations is the selection of an appropriate physical property method that will accurately describe the phase equilibrium of the chemical system. Missing or inadequate physical properties can undermine the accuracy of a model or even prevent one from performing the simulation. For this reason, finding good values for inadequate or missing physical property parameters is crucial to a successful simulation. Nevertheless, this depends strongly upon choosing the right estimation methodsāan issue already recognized in the world of chemical processes modeling by the axiom āgarbage in, garbage outā which means that the simulation results have the same quality as the input data/parameters (Carlson, 1996). In most design situations there is some type of dataāfor example, VLE reported in the literature, experimental measurements, and data books (Gmehling et al., 1993; Perry and Green, 1997)āthat can be used to select the most appropriate physical p...
