120260 Shortcut Distillation

120260 Shortcut Distillation - Reactions and Separations...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
60 www.cepmagazine.org December 2002 CEP Reactions and Separations t is often necessary to develop data for a range of operating conditions, so that the optimum con- figuration of a distillation tower can be found. There are two conventional methods to perform such a task, either graphically by hand (which is some- what inaccurate and time-consuming) or by any of a number of commercial simulations that are faster, but costly to license. A third alternative is presented here: Merging the graphical and manual computa- tional methods so that the inaccuracies of the former are compensated for by the speed of the computa- tions. The calculations can be run by any spreadsheet program, such as Microsoft Excel, eliminating the need for employing expensive simulation software and for laboring over hand calculations. Further, the time involved from a programmer’s point of view is no more (or considerably less) than that required to learn how to use a commercial simulation package. The method presented here is easy to learn, and offers a quick way to make preliminary estimates of the tower diameter and height, number of stages, energy consumption, and reflux ratio. Although the calculation procedure is intended for binary sys- tems, ternary systems can also be modeled if the third component is less than 10% by volume and its volatility is not drastically different from the those of the remaining two components. Spreadsheet calculation procedure The first step in any binary distillation calcula- tion (apart from performing the mass balance around the column), is determining the vapor/liquid equilibrium (VLE) data. Raoult’s law is used to cal- culate the saturation pressure for the pure compo- nents. Since most systems are non-ideal, the Van Laar equation is then applied to determine the liq- uid and vapor compositions. This equation includes the activity coefficients for a mixture, making it suitable for non-ideal systems ( (1) , p. 32). An ethanol/water system is used to illustrate the overall method. Apply Raoult’s law and the Van Laar equation Use Raoult’s law to find the saturated vapor pres- sure for each component. Published data are avail- able for various compounds ( (2) , p. 10-141). The sat- urated vapor pressure of each component is expressed as: P 1 sat = 10 [ A B /( T + C )] (1) where P 1 sat is the saturated vapor pressure of compo- nent 1 (mm Hg), T is the temperature (°C), and A , B and C are the Raoult’s law constants for each com- pound. For ethanol, A = 8.32109, B = 1,718.1 and C = 237.52. The values for water are: A = 8.07131, B = 1,730.63 and C = 233.42. To calculate the saturation pressure of a component, simply substitute the value of the temperature. In these calculations, 1 refers to ethanol and 2 is for water. This method uses a numerical solution to a McCabe-Thiele diagram to find the theoretical number of stages for binary and pseudo-binary systems, then calculates the actual number of stages, reflux ratio and column dimensions.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 8

120260 Shortcut Distillation - Reactions and Separations...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online