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Journal of Chemical Engineering of Japan, Vol. 41, No. 9, pp. 000–000, 2008 Short Communication Copyright © 2008 The Society of Chemical Engineers, Japan 1 McCabe–Thiele Method Revisited—Solving Binary Distillation Problems with Nonconventional Specifications Luís Gonzaga Sales VASCONCELOS 1 , José Jaílson Nicácio ALVES 1 , Antonio Carlos Brandão DE ARAÚJO 2 and Romildo Pereira BRITO 1 1 Depa rtme nt o f C hemi cal Eng ine erin g, Federal University of Campina Grande, Av Aprígio Veloso, 882, Bodocongó, Campina Grande-PB, 58109-970, Brazil 2 School of Engineering, Laurentian University, 935 Ramsey Lake Road, P3E 2C6, Sudbury, ON, Canada Keyw ords : Distillation, McCabe–Thiele Method, Undergraduate, Mathcad™ At undergraduate level, the sep aration of binary mixtures is studied using, mai nly, the McCabe–Thiele method because of the advantage to present the results in graphical form. However, most of the applications are limited to problems where extreme (top and bottom) compositions and reflux ratio are specified and the objective is to determinate the number of stages. By the use of the McCabe–Thiele method and the advantage of a solve-package (Mathcad™), a procedure is proposed to solve problems with different specifications. Unlike the traditional application, the number of stages can or can not be specified. De- pending on the available information, the objective can be one of determining top or bottom composition and reflux ratio, for instance. The proposed procedure allows for detailed analysis of the separation, avoiding the cycle “right solution or wrong solution”. The proposed procedure is also valid for columns with multiple feeds and can be implemented by using other software. Introduction Distillation is one of the most important separation processes used in chemical and petrochemical in- dustries due to its great efficiency in separating different types of mixtures. Therefore, chemical engineering curricula cover one or more courses involving this subject, which always consider binary mixtures to be- gin with. As computer codes have become more popular, solutions to problems involving distillation calculations have been approached by using more powerful and ef- ficient software (Aspen™, Hysys™, Design™, etc.). However, graphical methods for dealing with problems like those provide a preliminary overview followed by a deeper understanding of the distillation process, which are fundamental pre requisites to initiate stud- ies on the separation of multi component mixtures. Besides, the software mentioned above are a very ex- pensive part of any budget such that not everyone can have access to those tools. Among all graphical methods, the most popular are McCabe–Thiele and Ponchon–Savarit methods Received on February 8, 2008; accepted on April 28, 2008. Correspondence concerning this article should be addressed to R. P. Brito (E-mail address: [email protected]). (King, 1971; Henley and Seader, 1981; Kister, 1990). These methods differ from one another because the later considers the energy balance by using enthalpy diagrams. At the undergraduate level it is common to consider ideal phases, both liquid and vapor, in the process, which makes the supposition of constant mo- lar flow throughout the section true and turns the McCabe–Thiele method into one of the most widely used methods. In most of the applications involvi ng the McCabe– Thiele method, if the characteristics of the feed (composition and thermal condition) to the distillation col- umn are given, top and bottom compositions are specified as well as the reflux ratio. Another very common application of this method occurs when the recovery ratio of one of the components, the specification of bottom or top composition, and the reflux ratio are given. That is, traditionally the McCabe–Thiele method is used just to determine the number of stages to achieve a specified separation. The fact that the McCabe–Thiele method has been used mainly to determine the number of stages is due to, in the recent past, problems involving this subject having been solved by hand (means of grid paper). For a given problem, the equilibrium curve along with the operation lines was plotted, and then the number of stages was obtained. All of those tasks were done manu- ally. For every new operational condition like, for

jcej08we024

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